Welcome to Android Central! You somehow powered on into Download Mode (aka Odin Mode). This is usually achieved by pressing and holding Power and Vol Down at the same time when powering on. Download Mode is used to transfer a firmware file to the tablet (which you don't need to worry about at this time), so as long as you're not connected via USB to a computer, it's safe to exit that mode. Is pressing the Power and Vol Down button not working to get you out? You may have to press those buttons together and hold them down for 10-15 seconds to force a shutdown.
Once you're able to shut it down, power on by pressing the Power button only. If it gets into Download Mode again, there may be a problem with the buttons--for example, the Volume button might be stuck in a pressed state.