In audio it refers to the boosting/cutting of specific frequencies of the audio spectrum, and if you're tracking things correctly you shouldn't need to use much if any.
There are two basic types of EQ: parametric & graphic. A parametric EQ has controls for the freq. of the cut/boost, the width (or "Q") and the amount. A graphic has individual controls for selected freqs that work on a + or - scale. You've probably seen the latter on PA's, stereo's etc. There is also low/high pass ( cuts all freqs below or above a certain point) and shelving ( boost or cut everything below/above a set freq- common on stereos & cheap mixers).
Better to get your sounds as close as possible to what you want before reaching for that EQ. It adds distortion, and phase artifacts to the sound in the process of changing the spectral balance. The cure is often worse than the disease.
Of course there may be certain situations where radical EQ settings may be desireable. That depends (there's that term again) on what your sonic goal is. There are also times when a bit of surgical freq. removal is required to make an instrument sit well in a mix.
IMHO it's best to use as little as possible, and if you don't need any that's great!