From MEPIS Documentation Wiki
grep is a command line utility that searches for lines of text that match one or many regular expressions, and returns only the matching lines. The most common use of it is to pipe the output of another command into it so you can find something in particular, as grep will print out any lines which contain the string you give it.
For example, this will print out all the entries from fstab related to hda.
cat /etc/fstab | grep hda