A colloquial form of 'この間' meaning 'the other day', 'recently' — used to refer to a past event that happened not long ago.
こないだはありがとう。助かったよ。
Thanks for the other day. You really helped me.
こないだ映画を見たけど、とても面白かった。
I saw a movie the other day; it was really interesting.
あの人、こないだ会ったばかりなのにもう忘れたの？
We just met that person the other day—have you already forgotten them?
こないだの件について、また連絡します。
I'll get back to you about the matter from the other day.