politely postponing or declining an offer for now, but suggesting you'd like to accept it another time

Do you want to grab dinner tonight?

I'm busy, but can I take a rain check?

Meaning that not tonight, but let's do it another time.

Where it comes from?

The phrase originally comes from U.S. baseballl in the 1800s.

If a game was rained out, spectators were given a "rain check" - a ticket to come back when the game was rescheduled.