In an attempt to explain in layman's terms what's going on here:
The last two bright-white lines are alerting you that the disk wasn't properly unmounted... this usually means a crash (panic) or a power outage. However, now that we have background fsck (the unix-like equivalent to "scandisk" or "disk doctor" or any other equivalent program you can think of), that's no longer a fatal condition... the boot can keep going, and fsck can run in the background in most cases.
So your boot continues for a bit, until you get to the "Starting Filesystem checks:" line, where fsck (filesystem checker) realizes that the particular issue with your filesystem isn't something that can be fixed in the background, it needs to be fixed now... so it stops the boot process (sends sigterm to it's parent, to say to rc "terminate the boot process").
rc's standard reaction at this point is to give you a prompt to select a shell for "single user mode" so you can fix the issue. As stated above, pressing enter at that prompt should give you a # symbol, which is an indicator of a root shell... from which you should be able to run the command:
# /sbin/fsck -y /dev/ad0s1a
After it's done, pressing Ctrl+D should continue the boot process... if not, a reboot will do the trick. If on the next boot another partition has a show-stopper inconsistency, repeat the process. If on the next boot the same partition has a show-stopper inconsistency, you likely have some dead hardware.