This plugin checks the health of a ceph cluster.
The plugin usage is given in the source code as the following:
Usage: check_ceph [-m|--mon] [-k|--key] -m, --mon=ADDRESS[,ADDRESS,ADDRESS] IP address(es) of ceph monitors -k, --key=string secret key to access the ceph cluster -h, --help Print detailed help screen
You can read about Nagios plugin development here:
http://nagiosplug.sourceforge.net/developer-guidelines.html
In particular, you will likely be interested in learning about plugin output.