Equalization is the most fundamental mixing tool. It allows you to boost or cut specific frequency ranges in an audio signal, shaping its tonal character and helping it sit in the mix.
The frequency spectrum is divided into ranges: sub-bass (20-60 Hz), bass (60-250 Hz), low-mids (250-500 Hz), midrange (500 Hz-2 kHz), upper-mids (2-4 kHz), presence (4-6 kHz), and brilliance/air (6-20 kHz).
Common EQ moves include: cutting low frequencies from vocals to reduce rumble, boosting presence frequencies to add clarity, and cutting competing frequencies between instruments so each occupies its own space in the mix.