Complementary events are “opposite events“. In probability, the event of success (the event happens) or failure (the event not happening) can be classified as a complementary event. For example, the chances of rolling a 5 and the chances of not rolling a 5 are complementary events.
The probability of complementary events always add up to 1 (for 100%), because there are no other options.
For example, you can be blue or not blue (blue’), you can be an even number or not an even number, but nothing else!
A very common misunderstanding is that events have to be equally likely. In reality, this does not necessarily need to be the case.