It is a phrase that means "rarely". It is used when you almost never do something or when a thing doesn't occur frequently.
ケンはめったに学校を休まない。
メグはお酒が弱いので、めったに飲まない。
- 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 : 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。