In statistics, complements mean the opposite of something. So the complement of “probability of seeing a heads in 2 coin flips” is “probably of not seeing a heads in 2 coin flips”. This is setup only for mutually exclusive options.
The Rule of Complements says the probability of the event and the complement should sum to one, given they’re mutually exclusive.
Sometimes, determining the event, complements are easier to determine, so you can reference that as .