Equalisation (EQ) was developed around the signalling and playback medium of digital audio tape (DAT) as a means to adjust the gain of specific frequencies in a signal. It has come from being used to simply lower noisy frequencies by above or below 6dB in the low-end and high-end, to being a complex system where engineers often use EQ on potentially infinite frequencies as a means to get the most realistic or immersive experiences from their sounds.
A sophisticated development in EQ is Dynamic EQ where a specific frequency is automated to boost or lower the gain depending on how the engineer programs it to work around other frequencies in the track.