What is a 'blue moon' in the calendar sense?

In one line

The rule

A blue moon is the second full moon in a single calendar month.

Let's understand it simply

Imagine you see a full moon at the start of May. Usually, the next full moon comes after about 29 or 30 days. But sometimes, if the first full moon is right at the beginning of the month, there is just enough time for a second full moon before the month ends. That second full moon is called a "blue moon."

Remember it

A blue moon is just a rare second full moon in one month—not a blue-colored moon!