It is used to filter the output from the show command to only include the bit you type after the 'inc'. For example, if I have a large config, and I need to see all the 'ip route' commands only, the a 'show run | inc route' command will show all the ip route commands. It will also show any route map commands as these lines also include the same word. I usually find that the '| begin' is more useful, as it is easier to start at the right point. Both are useful when needed, but are most used when the config is large.

Hope that explians it a little. Just get used to using it and you will discover how useful it can be.