Explain ISAs to me

[Mercury00]

I opened an ISA account a few years ago with Santander but I've never had any kind of statement or paperwork from it so I have no idea of how to use it, when the money has to paid in by, allowance limits etc. I really have no idea what I'm supposed to do with it. Can someone explain in simple terms how much I can deposit, until what date each year, and what I do with it after that?

P.S. Sorry for being stupid