EQ, at it’s heart, is based on a very simple idea:
Sounds with the same frequency content sound the same
The more two sounds share similar frequency content the the more similar they sound. Any sound is made of a collection of vibrations at different frequencies and different loudnesses. This is true of everything from a whistling sound (an almost perfect sine wave, a pure single frequency) to a symphony (an infinite set of frequencies).
The next principal that’s important:
Matching frequencies add up
If two sounds share frequency content (put another way: if the share frequency space) those shared frequencies will add up. They’ll get louder when played together. This is everything you need to know to understand EQing.
Two sounds will ‘compete’ if they share frequency space. You can use EQ to remove frequencies from one sound so that it doesn’t compete with the other.