The command "fsck" stands for "File-System CHeck". It is run at boot time to make sure the disks don't have errors (usually caused by not shutting down properly). When you are booting your system, it is saying, "Hey, there is a file-system that isn't working properly, why don't you run fsck manually and fix any errors."
Don't try go mount the corrupted file-system, fix it.
This can be done by running "fsck -y /dev/sda2". If you were more experienced, I might suggest not using the -y and drive fsck manually, but since you are new to fsck, just let it try to fix things automatically.