A bit is a measure of information theoretical entropy. Specifically, one bit has been defined as the uncertainty of the outcome of a single fair coin flip. A single less than fair coin would have less than one bit of entropy; a coin that always lands heads up has zero bits, n fair coins have n bits of entropy and so on.
https://en.m.wikipedia.org/wiki/Information_theory
https://en.m.wikipedia.org/wiki/Entropy_(information_theory)