When do you say it? How would you say it another way?
It'll keep.

Question

Answer

Like Colin said it's colloquial. I'd use it to talk about food. 
"There's bread in the cupboard. It'll keep until ('til) tomorrow"
In this context you can also say it will last, or it won't go off, or the date (BBE/Use by) is okay until...

Answer

This has lots of meanings depending on context. It is a colloquial expression, so you could say: 'We can talk about it/do it later.' Or 'We don't need to talk about/discuss it now.' Really, there are a lot of expressions that would be more specific.