Attached are some documents on coherence.
It is the cross-spectral similarity between the channels, scaled as a percentage. 100 means the signals are identical, in the frequency band of interest. 0 means they are orthogonal, hence totally different. Even random signals are typically 30 to 40 percent coherent. It is like a Pearson Correlation Coefficient. It is calculated based on the spectral energy, and is done inside the digital filters.
It is basically a measure of how much similarity or communication there is between the two channels.
Physiologically the coherence is proportional to the number of connections and the strength of the connections.