i had an error at Ubuntu 17.04 before booting. It said: "UNEXPECTED INCONSISTENCY, RUN FSCK MANUALLY" . So i typed after finding the problem's solution in Google: fsck /dev/sda2 . After rebooting my System worked normally without any errors. My question is: what does fsck /dev/sda2 mean?
fsck is the linux file system check command. You can use it to check if there are any errors or attempt to repair errors in the file system. man fsck to learn more about that.
dev/sda2 is the drive which is causing the error. The a part refers to the first disk, so most likely the one you're booting from and the 2 part means the second partition.
Without anymore details it's quite hard to tell what the actual problem is. Possibly it's a corrupted drive or that it is dying. If possible try to back up your data from it.