Others have already mentioned man grep
or grep help
But, in case you don’t know about it, there are two great utilities to get examples and help for almost any given command: tldr
and cheat
are great.
https://github.com/cheat/cheat
https://github.com/tldr-pages/tldr
Just cheat grep
or tldr grep
and you’re good to go :)
Ex-Machina and I, Robot are just so good