what's the meaning about "rain cheque "?

I assumed you meant Rent Cheque.
In United States, you find something from a department store with very attractive price, unfortunately, it ran out of stock for the time being, so, you are entitled to ask the store to issue a Rent Cheque. Which would allow you to pay the same price as the moment it was on sale whenever stock is available in future.

1. promise that an unaccepted offer will be renewed in the future

2. A ticket stub entitling the holder to admission to a future event if the scheduled event was cancelled due to rain