Put very simply, EQs enable you to cut and boost certain frequencies of your music, either track by track (typically when mixing), or across the entire mix (when mastering). They are probably the most important mix processor you can turn to, and many producers and engineers advocate setting an EQ on every channel. You can use DAW templates to automatically load them in when starting a project.
You'd typically use an EQ to reduce the harshness of cymbals, take away bass muddiness from a mix by cutting the bass out of every instrument rather than the bass and kick, or reduce sibilance in a vocal (with precise EQing). But really, there are as many EQ applications as there are EQ types, which we'll deal with now.