EQing (equalization) involves changing the volume of different frequencies in your sound. This allows you to make huge (or tiny) changes to the sound of your tracks. This is an incredibly valuable tool in audio mixing.
Try right clicking on this plugin in your software of choice and dragging all the way down then moving your mouse left and right while the sound plays. See what changes are made to your sound when you cut certain frequencies. This is a good way to get a feel for which frequencies make up each part of your sound.
Here are some terms you should know. Some are pretty self-explanatory. You don't need to read through all of these now, but this will be here to look back on when I use these terms later.
Bass - The lower frequencies. The kind that make the earth rumble. Yeah, it sounds cool when you turn your bass up too high, but don't be a jackass and crank the bass in every one of your mixes. Uncontrolled, a lot of bass ends up sounding like shit.
Body - The upper bass frequencies.
Boomy - Way too much fucking bass. Back off a little.
Bright - Lots of high frequencies.
Fat (big, full) - Takes up a lot of space in your mix. Sounds badass by itself, hard to mix. Often includes some stereo tomfoolery that makes the sound bigger.
Harsh - Too much upper mid.
Mid - That under-appreciated area between bass and treble.
Muddy - Shitty sounding because too many sounds are competing for the same frequencies.
Muffled - Low upper mids and highs, sounds like you're singing into a styrofoam cup.
Smooth - Easy to listen to, all the frequencies are more or less equally loud.
Thin - The opposite of fat.
Tinny - Sounds like it's coming through bad speakers, like a telephone.
Treble - The higher frequencies. Turning these up might make your mix sound clearer, but it will hurt your fucking ears if you get carried away.
Warm - Plenty of full mids and bass.
Coming soon - EQing advice more relevant to mixing multiple tracks.