Just went under contract on a house, what's next?

So we were pre-approved, just went under contract on a home and wired over our deposit. What exactly is the next step in this process? I've been told it'll go off to underwriting but what exactly does that involve? When should I know if I am actaully approved or not and what can go wrong during this part of the process?

This is our first time buying a home and we did a lot of work to get our finances to a reasonable place for pre-approval so we're just nervous that something will go wrong. Any advice is appreciated.