Inbox problems, please help me.

Hello all,
I have been using outlook for ages now with no problems at all then yesterday I opened it while not connected to internet and an error message regarding .pst file? Saying I should repair it but I have no idea how to do this. What the hell is an inbox repair tool
Luckily, I had only just backed up so I copied over that file and it seems fine again now I was just curious as to what exactly the problem was so I can address it in the future should if I need to.
Any help is appreciated!
