What does this start up screen mean?

13" MacBook Air, purchased Nov 2015.

No prior issues. Computer was powered down, I was carefully (or not) wiping down the keyboard, accidentally pressed power key and goodness knows what other key. Computer made its usual power up sound, then THIS screen came up. I have no idea what to do with it.

I'd really like to get my usual startup/login screen back. Can I just power down again?

I'm stupid about how computers work... this screen scares me. Help!

Thank you.

(I have updated El Capitan, but I don't know whether I have the most recent update.)