In simple terms, a compressor compresses the louder parts of an audio signal and boosts the quieter parts, resulting in a more consistent level of volume throughout a song.

Sidechaining is a production technique used in a wide variety of music genres where the volume of one track, influences another track.

Gating is turning on or off one sound source completely, in response to another signal. It is particularly useful for giving an otherwise mundane sound a dramatic rythmic effect.

"Effects ducking" refers to a technique where the volume of certain audio effects (such as reverb or delay) is automatically lowered when another audio track (usually lead or vocals) is playing.

The Dynamics ('dyna') page gives you comprehensive control over the dynamics of your track; per-track sidechaining, gating, effects ducking and compression.