The phrase "rain check" is an idiom that is often used in informal situations to mean postponing an invitation or offer to a later date. Specifically, it means that the person cannot accept the invitation or offer at the current time, but would like to do so at a later time or date.

The phrase is thought to have originated from baseball, where a "rain check" was a ticket issued to fans when a baseball game was cancelled due to rain. The ticket allowed the fan to attend a future game in exchange for the cancelled one.

Here are some examples of how "rain check" can be used in sentences:

1. "I'm sorry I can't make it to your party tonight, can I take a rain check and come next week instead?"
2. "I was really looking forward to going to the concert with you, but I have to work late tonight. Can we take a rain check and go next month?"
3. "I'm sorry I have to cancel our lunch date today, can I take a rain check and treat you to lunch next week instead?"
4. "I'm sorry I have to decline your offer to go to the beach this weekend, can I take a rain check and join you next time?"
5. "Thanks for inviting me to your barbecue, but I have other plans this weekend. Can I take a rain check and join you at the next one?"

Overall, "rain check" is an idiom that means postponing an invitation or offer to a later date, and it is often used in informal situations.