Compression
A dynamics process that reduces the loudest parts of a signal so the overall level can be raised, evening out a performance.
Audio compression reduces the dynamic range of a signal — the gap between its loudest and quietest moments. A compressor watches the incoming level, and once it crosses a threshold you set, it turns down the volume by a ratio you choose (3:1, 6:1, 10:1, etc.). This lets you safely raise the overall level afterward, making the quiet parts more present without the loud parts clipping.
Five knobs do most of the work: threshold (where compression starts), ratio (how much it compresses past that point), attack (how quickly it clamps down), release (how quickly it lets go), and makeup gain (the volume bump after compression). Each knob is a craft decision. A slow attack on a snare drum lets the initial crack through; a fast attack flattens it into a thud. A slow release on a vocal smooths it; a fast release can pump unnaturally.
Compression also has a tonal effect. The hardware compressors of the 60s and 70s (1176, LA-2A, distressor) imparted color even at modest ratios, and modern plugins emulate that warmth. Used surgically, compression is invisible — it just makes a part sit right. Used aggressively, it's a creative effect (the drum-bus pump, the parallel-compression vocal lift, the EDM sidechain).