What he's telling you is that you basically have a few little 'pops' in volume that can be trimmed a little. These are going to come from the instruments mentioned more than anything else. They're basically the really fat people who sit on the bus and take up several seats on the crowded 334 so that a bunch of skinny people, who could easily fit three or four abreast in the same space, are forced to the aisle and a handful more are left out of the vehicle entirely. Your mission, should you choose to accept it, is to lop all but about 85 centimeters of the middle (try to keep the vitals, aye?) of these wretched lard-balloons (try to cauterize it on the spot, as well, because you can tell when blood stains haven't been properly taken care of) and position them in their proper places so that everyone can board and get where they want to go.
When you have as much of a 'jump' as these percussive hits do, and you leave it there, you leave a lot of bandwidth unused but have some obnoxious snaps popping up here and there - think a bunch of two-story houses and a random skyscraper thrown into the mix here and there; if you want to take a picture of the skyline (your mix) you're either going to have very little houses (axes, vocals, the like) and very out-of-place Sears Towers (kick, toms, and that horrid beast of a snare) or normal-looking houses (axes, vocals, the like brought up to a good volume) and all but the first few stories of the latter lopped off (kick, toms, and that horrid beast of a snare getting smacked back into place) - and your mix will not be getting all it can be in terms of volume. Clipping will let the hits keep their character and sound (unlike too much compression all on its own, which will go from unused to 'pumping' if you pass through once and chomp down too hard) so that they can fit into the mix better - they'll be closer to the same 'steady' volume as your other tracks and as a result you can bring them, and other things, up in the mix without having so much needle bouncing that just leads to perceived quietness *and* clipping at the same time.
In mastering, 99.9 percent of the time you'll have most of your stuff sitting pretty quiet, but then some obnoxious THWANK will pop you into the red and you'll wonder where the fuck it came from. If it wasn't for those smacks you'd have several decibels of gain to play with, but instead you're stuck where you're at because of those obnoxious hits... you think. If you have some really mean peaks, you can clip them lightly (or heavily, or in the middle, or whatthefuckever) and free up a little 'space' for everything else to come up. You have to be careful not to sound like you're just shaving off peaks, or drawing in right angles and straight lines with the pencil tool, but you'd be surprised what you can get away with before noticing fresh hell in your mix - try a light clip before you compress, and you'll not only get (1) an RMS a few decibels higher but (2) less 'pumping' out of your compressor... or more compressing before 'pumping', depending on what you want to do.
Jeff