Well that's the Fastboot menu as you probably knew seeing as how it's clearly written on the screen. That's the doorway into doing a number of things to the phone that can't be done from the main screens, including flashing another ROM (operating system), to it.
How you got that screen from a simple boot is the real question.
Have you tried rebooting it? You can hold down the power and volume down buttons together for 10 seconds and it should cold boot. Let us know if you get any different results.