You typically get asked to enter the path to a shell at boot-time if there is a configuration problem that freebsd detects and it determines from this it is best to drop you into single user mode so you can make the required modifications to the system configuration. Single user mode is analogous to windows 'safe mode', loading the system minimally so as to allow you to make the changes needed to get the system up.
For example the last time this happened to me was yesterday after I had edited the /etc/rc.conf file. I had mistakenly missed a closing " in one of my /etc/rc.conf entries and so freebsd, unable to parse the file correctly, dropped me into single user mode so I could make the changes to the file.
This required specifying the full path
to the vi editor/file:
from where I could make the necessary changes to the rc.conf file to allow me to reboot into multi-user mode.
READ THE ERROR BEFORE THE LINES ASKING YOU FOR THE PATH TO YOUR SHELL - you should get some indication of what went wrong and what you should do to fix it.
Note also that you may need to mount one or more of your drive devices first to fix the problem. Also you may need to remount the partition in rw mode (since freebsd mounts some partitions read only to avoid corruption).
To check what is currently mounted and in what mode, simply type: /sbin/mount at the command prompt (again using the full path to the mount executable because your path may not be setup in single user mode). If the partition on which the files you need to fix reside is not mounted (or mounted in read only mode), you will need to mount the partition using: /sbin/mount -t ufs <mount-point>, where <mount-point> is a valid entry in your fstab file (ie mount -t ufs /).