I assume this is coming after some other command, right? Such as cat somefile .
So, what the following pipeline command does
cat somefile | grep "^[a-zA-Z]"
is return all lines that start with a letter, whether capital or not. Effectivly filtering out all lines that do not.
That is, as long as it is double quotes ( " ) that are used. I believe that if it is single quotes ( ' ) are used, that it will only return those lines that contain (anywhere in the line) exactly those characters. I do not have a *nix system at hand, currently, to check that again though, and it has been a while since I have been able to work extensivly on an *nix system.