The standard signal strength of most DJ gear, as opposed to the much weaker phono level of turntables.
Line level is the signal strength output by most DJ equipment such as media players, controllers, and mixers. Turntables output a far weaker phono level that needs a preamp to reach line level.
Why it matters
Plugging a turntable into a line input, or a line device into a phono input, gives you either no sound or massive distortion. Knowing the difference prevents a confusing setup.
In practice
Set each mixer channel's input switch to match the device: phono for turntables, line for everything else.
