It means seldom. Or like so rare. Something that happens rarely or has the tendency to occur with a long gap, then this expression is used.

It means something that doesn’t happen often, like how a blue moon is very rare event to see.

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It means that something that happens very rarely or something that occurs once in a very long time. :)

"A blue moon" is a second full moon in the same calendar month. "Once in a blue moon," means once in a long while, rarely, seldom.