DSP

The foobar architecture is sufficiently evolved to allow the user to use simultaneously several DSP or effects.As its decoding and engine work in 64 bit floating mode, the loss are put away from the hearable field.
To activate a DSP, just put in in the left column (from Available DSP to Active DSP). The order of DSP has its importance, as a consequence take to the order (use the up and down button to do so, the chain proposed on the picture might be a good one). For a topic on the subject: click here. If you don't have a lot of CPU, put the resampler at the end for less processing. Always put the soft clipping limiter or the advanced limiter at the end, so as to manage sound enhancement and have useful measures against clipping.
DSP means Digital Signal Process. It isn't necessarily a real enhancement of digital datas: volume control is a DSP for example. The user can improve the sound or really decrease it if ill-managed. The following list is far from being complete as DSP avaible are numerous but I will just describe the most useful ones:

• Advanced limiter: it's a tool which evens out your tracks. So as to smooth peaks and put (only) them at a level which won't cause any clipping problem (could be useful with replaygain, even if the use of this option theorically avoids this problem). It's use is recommanded as its efficiency is high, but needs "a lot of" resources. If you see Monkey limiter or something like this in old documentation, it is this component
  You should put it at the end of your DSP queue.
• Soft clipping limiter: flattens out all clips behind a fixed limit. Advanced limiter should be preferred, for some topics about the difference between the two: click here or here. As far as I understood, the soft clipping limiter can only reduce the audio level by 6 dB while the advanced limiter can cut more than 6 dB if needed.
• Crossfeed: (not linked to crassfading) it is useful for headphone users. Because if headphones have a great accuracy for listening, the separate two much the stereo scene, creating audio tiredness linked to the intent use of your brain, it also decreases the natural aspect of the recording. Crossfeed should be used so as to use your headphones as if they were speakers with all of their qualities without the defaults quoted above.
  Desactivate it if you do not use headphones. For a better explanation go to this site: Meier Audio
• Dynamics compressor: it levels the audio level of the different parts of the track, It's not good from a listening point. It can soft the transition into a track, as a consequence, it could be used for the diskwriter (for portable devices use) (for an image of the panel, click here
• Equalizer: it has 18 cursors, which makes it very accurate. It has been created to compensate hardware defaults of your hi-fi components. presets are avaible from here
• Gap killer: foobar is totally gapless, but this DSP is here to correct mp3 or AAC defaults. The gaps are short silence inserted artificially, it could last a tenth of second but could be hearable if applause are splitted on two different tracks.
• Pause between tracks, the invert of gapkiller, inserts pause. useful for MD users, You can customize it into the corresponding panel.
• Resampler (SSRC): it will resample better than the audio cards drivers your music . The slow mode is not recommended as it uses more resources. Users who have audio cards supporting high sampling frequencies (88200, 96000, 192000 Hz) won't have a real improvement from 44100Hz but could be useful to add "dead" datas. Resampler is useful especially for low frequencies sampled tracks (radio recorded for example). Just remember that the audio hearable zone of humans is from 20Hz to 20000Hz and that the high harmonics have lesser impact as their frequency is high (CD are sampled at 44100Hz).
• Resampler (PPHS): the same as above, but just a different technique (click here for more details about PPHS)
• Crossfading: It mixes the last of a song with the start of the next one so as to have some continuity, you can change the length. Not that it could be amazing if after a hardrock file you shift to a vocal track.. So be careful for its use
  
• Convolver:It serves to model your acoustic environment (stadium, hall, room) if I have understood but I didn't understand everything of what it does, so this is a link
• Reverse stereo channels: puts which should be the right channel to the left one and vice versa.
• Simple surround: adds surround infomation to your file (it expands the stereo image)
• Sound touch: accelerates or slows down the tempo of your tracks. Use with care!! Useful to slow down the pace for music players who want to have the notes or play the track a bit slowly than the performer is able to.
• Skip silence: detects silence and cuts it. You can change the minimal length to cut and the threshold in the corresponding panel. It is not the same as Gap killer, it only cuts long silences into a file (not short silence between two tracks). It is useful for bonus tracks for which there are several minutes of silence before having it start, it will become only few seconds with this tool.
• Channel mixer: Will replace the several downmix or upmixing DSP like downmix channels to mono, convert stereo to 4 channel, convert 5.1 to stereo, convert mono to stereo channels. Additional component for a complete detail of its ability. click here
• Continuator:could be used instead of crossfader. Additional component for a complete detail of its ability, click here

There are several other DSP, but as the foobar community is very active, it is hard to have a review of each one. I've only described the most common ones. To download extra DSP, go there.