It is a phrase that means "rarely". It is used when you almost never do something or when a thing doesn't occur frequently.
Ken is rarely absent from school.
ケンはめったに学校を休まない。
ケンはめったに学校を休まない。
Meg hardly ever drinks because of her low alcohol tolerance.
メグはお酒が弱いので、めったに飲まない。
メグはお酒が弱いので、めったに飲まない。
Meg and Amy are talking about Father's day.
- Meg : Hey Amy, what do you usually do for Father's day?
父の日は何するの？ - Amy : Well, I normally go back to my hometown to visit my dad and take him out for dinner.
んー、地元に帰って、お父さんと夕食を食べに行く。 - Meg : Wonderful. How often do you visit your family?
いいね。よく実家に帰るの？ - Amy : Honestly, once in a blue moon. My parents' house is in the countryside so it's difficult for me to get out there often.
正直(に)言って、めったに帰れない。田舎だから、頻繁には行けない。
Ken has handed over his assignments to Leo for tomorrow while he's out on a business trip.
- Ken : I already finished describing what you should do while I'm away. If you have any questions now is the time to ask them.
出張中にやってもらいたい事は伝えたし、引継ぎは完了。何か質問はある？ - Leo : No, thank you for your help.
ううん、ありがとう。 - Ken : You're welcome. I never thought I would be chosen for this business trip.
どういたしまして。まさか、この出張に僕が行けるなんて思わなかったよ。 - Leo : Consider yourself lucky. An opportunity like this is something not everyone gets to experience.
いいな。こんな機会はめったにないよ。