An adverb meaning 'rarely' or 'seldom'. It is normally used with a negative verb (e.g., 'not') to mean 'hardly ever'.
彼とはめったに会わない。
I rarely see him.
彼女はめったに遅刻しない。
She hardly ever arrives late.
めったにないチャンスだから、挑戦してみたほうがいい。
It's a rare opportunity, so you should try it.