Equalization refers to the process of adding or subtracting frequency information. A simple EQ can also offer the ability to filter frequencies as well. That being said, filtering is also a method of equalization except filtering operates on frequency rejection.
Drum equalization differs slightly from full mix equalization as you are only presented with the drums at first. Once you begin working on the mix as a whole song, some further equalization may be needed to sit the drums a little better within the entire context of the mix.
A high pass filter is almost always necessary on drums as transients a dramatic amplitude climbs can produce digital artefacts simulating sub frequency content as well as super-sonic frequency content that can be taken care of with a low pass filter.
For dominant drums such as the kick, clap, snare, 808’s and some tom drums, seeking the fundamental frequency of the lowest note and using that as the cutoff for the HPF can help clean up messy sounds by removing unwanted rumble.
Another method of equalization, especially for kick drums is to control some muddy frequencies by scooping out just 1 or 2 dB. This range will inevitably build up so controlling this band early can help with the mix further down the line. Often a move like so will enhance the fundamental frequency even further.
Cutting frequencies away with an EQ is far cleaner to mix than fighting with EQ’s that have been pushed to fight opposing elements. Finding frequencies is easy with a tight and narrow Q (resonance factor) and thus can be removed with ease.
For high frequency instruments such as hihats, cymbals and crashes, using more extreme high pass filtering may be necessary as information below 900Hz becomes obsolete when dealing with cymbals. This allows more room for the low frequency drums to operate while simultaneously removing irrelevant, muddy frequencies. Don’t be afraid to create a low pass filter around 19kHz should any cymbal create a sharp, piercing sound.
Sometimes drums benefit from an EQ boost, but be gentle this early in a mix as there are many more elements to come.
For darker sounding drums, try using a high-shelf to attenuate the upper end of the spectrum or vice versa for a bright, articulate drum!