The term "blue moon" has two meanings. The first and most common is when a blue moon occurs, which means it's the third full moon in a season or year that has four full moons instead of the usual three. This happens roughly once every 2-3 years.
However, there is also another definition of "blue moon" as a term used to describe something rare or unusual. For example, if someone says they've done something once in a blue moon, it means that they have done it very rarely or only occasionally.