GREP is a Linux command line utility for searching named input files (or standard input) for specific string patterns.
I would not go in detail about this utility, as there are excellent resources available on the internet already.
Below are the few sites that do an excellent job at providing a detailed explanation about this very useful utility.
Also, there is an excellent Free (Yes, Free) video tutorial at Udemy.
You just have to do a free signup and you are ready to take the tutorial at your convenience.