The amount you pay per page printed, typically fractions of a penny for black and white and a few pence for colour. It covers toner, servicing, parts and engineer call-outs.
A click is one printed side of A4. The click charge is what you pay for it, and on an inclusive agreement it covers far more than ink.
Black and white clicks are dramatically cheaper than colour — often by a factor of ten. A page with a single small colour logo on it usually bills as a full colour click, which is why colour costs creep up unexpectedly.
If most of your colour printing is internal, setting the driver default to mono is the single easiest saving available.