What is slew rate for I2C?
Slew rate is how fast the signal changes from low to high, or vice versa.  By limiting this abrupt transition, you can reduce ringing from signal reflections, and limit crosstalk between signal lines.

The way it works out, though, is that at 100kHz, the signal rates are so slow that the slew rate doesn't really matter; at 400kHz you may be able to fix an otherwise problematic circuit by limiting it; but then when you get to 1MHz you really need all the transition speed you can get, and so you just have to do good signal matching and route your lines more carefully.  

The bit in question does nothing more than enable or disable the feature.  The rest is simply advice.  The speeds are in parentheses, as it is just a suggestion.