What is restarting the bootloader done for, in Fast boot mode?

As the name implies, it'll restart the bootloader. To the get a better understanding of what lies behind Android's bootloader menu you might want to check out this tutorial.

Seems like other users had the same problem, in this page are listed some methods that could work, how I'd go about it all depends on whether you need to keep the data or you're just good with making your tablet work again. Wiping Cache and tapping the back of the device are both quite safe and you can simply give them a try. The other 2 methods listed are more destructive, in the sense that though Wug's Toolkit and factory resetting may solve the issue, all unsaved data will be lost if not backed up before-hand (I checked the FAQ and the Toolkit does actually have a "No Wipe Mode" option, but I'm not sure it can apply here).

For any other questions feel free to ask, and let us know if any of these methods worked for you