“I'll take a rain check on that drink” is an idiom that means you can't accept an invitation right now, but you'd like to do so at a later time. For example, you might say this if someone asks you out for a drink, but you have to work late: 
You: "I'll take a rain check on that drink. I have to work late tonight"
idiom: a rain check | English Help Online's Blog
The term "rain check" comes from baseball, where it was common in the 1880s to offer paying spectators a rain check if a game was postponed or ended early due to bad weather. The rain check entitled the spectator to future admission to the game. 
TAKE A RAIN CHECK (ON SOMETHING) - Cambridge Dictionary
used to tell someone that you cannot accept an invitation now, but would like to do so at a later time: Mind if I take a rain chec...

Cambridge Dictionary
To take a rain check☔️ (EXPLAINED) - YouTube
Oct 4, 2022 — can I take a rain check can I take a rain check what does this mean to take a rain check is an idiom that people use wh...

Idioms and Phrases This term comes from baseball, where in the 1880s it became the practice to offer paying spectators a rain chec...