In emergency mode, you are booted into the most minimal environment possible. The root file system is be mounted read-only and almost nothing is set up. The main advantage of emergency mode over single-user mode is that the init files are not loaded. If init is corrupted or not working, you can still mount file systems to recover data that could be lost during a re-installation.
To boot into emergency mode, use the same method as described for single-user mode in Section 11.3 Booting into Single-User Mode with one exception, replace the keyword single with the keyword emergency.