With Peachy permission, I extract it from my dictionary: a rain check (on something) 1. a piece of paper allowing one to see an event—which has been canceled—at a later time. (Originally said of sporting events that had to be canceled because of rain. *Typically: get ~; have ~; take ~; give someone ~.)  The game was canceled because of the storm, but we all got rain checks on it.  I can’t use a rain check because I’m leaving town for a month. 2. a reissuance of an invitation at a later date. (Said to someone who has invited you to something that you cannot attend now, but would like to attend at a later time. *Typically: get ~; have ~; take ~; give someone ~.) We would love to come to your house, but we are busy next Saturday. Could we take a rain check on your kind invitation?  Oh, yes. You have a rain check that’s good anytime you can come by and visit. 3. a piece of paper that allows one to purchase an item on sale at a later date. (Stores issue these pieces of paper when they run out of specially priced sale merchandise. *Typically: get ~; have ~; take ~; give someone ~.)  The store was all out of the shampoo they advertised, but I got a rain check.  Yes, you should always take a rain check so you can get it at the sale price later when they have more.

I had to look up the background. I knew it had something to do with rain delaying an event! :) It originally meant: "...at games that were postponed because of rain, spectators would receive a check (ticket stub) that could be used to attend a future game." Nowadays, it means a promise of a meeting later if it is not possible to have the meeting now: "I can't meet for dinner tonight, can we take a rain check?" (= can we meet at a more suitable time?) I think it's also used to promise a product later, if the product is not immediately available. ie. "the store didn't have the iPad in stock, so I took a rain check (or, they gave me a rain check)"