Jupiter
"Rolling off the bass [or kick]" just means attenuating (reducing) the level of those instruments (or others) at the lowest frequencies in a gradual manner.
This can help "clean up" a mix (although too much lo-freq roll off can leave your mix sounding thin).
Low frequency sounds require more power to produce and, as a consequence, having "too much" lows can "rob" overall volume to reproduce the very lowest frequencies -- which, in all likelihood, can't even be reproduced by most consumer or car stereos.
(If you've ever had a mix you couldn't get 'loud enough' no matter what you did, chances are it had 'hidden' lo-freqs that were stealing a lot of power.
Rolling off (often by use of a low-frequency shelving filter -- also called a
high-pass filter [because it lets the high freqs pass]) below, say, 30-
35 Hz can help give you enough 'headroom' so that you can then boost the overall level more.)
Of course, you can 'roll off' the highs, too (by way of a
low-pass shelving filter).
(Other EQ tricks involving low frequencies include manipulating the frequency curves of both bass synth/guitar tracks and kick drum tracks so that they complement each other.)