It's when you eq tracks to make the most of a certain frequency range so that they take up less space. A drastic example would be a funk band with horns. Bass would be all lows, rhythm guitar might be mostly 300-600 with the lead guitar mostly 800-1200 and the horns filtered with no lows (picked those frequencies at semi-random, btw). Then we get to find a place for drums, vox, and keys

. Drums are the hardest as the each drum has its own unpredictable character and cymbals have an off-the-charts bandwidth and can be finicky to eq. Experiment! If you're not sure where to start, there are some charts online with tips and general advice about different frequency ranges and how they apply to various instruments but it may take some google fu to find them. Or you just were using a different nomenclature and I wrote the great american novel for nothing.