By and large "fast" or "slow" bass can be roughly correlated with "tight" or "loose" and/or "clean" or "wooly" - a headphone with "fast bass" will have low-end that both attacks and decays quickly, and that isn't bleeding out over the rest of the spectrum, while "slow" is usually going to correlate with an overblown low end that both dominates the presentation, and hangs over the sound like a cloud.
An example of a "fast bass" headphone would be the Sony MDR-SA5000, and a "slow bass" headphone would be the Beats Solo (these are pretty extreme examples, usually where these threads turn south is when one "camp" says something like the K701 is "slow" and another says "fast"). But remember that as with most "audio terms" they're better used relatively as opposed to absolutely.