To complain is to express your unhappiness about something. You might complain to your landlord about a raise in your rent, or to the cops about your noisy neighbors. Just don't whine; it won't get you anywhere.
Speakers of Yiddish will know that complain is often interchangeable with kvetch, which is what you do when you don't like the food or the small portions. But you can complain in a more formal way by, say, writing a letter or even filing a lawsuit. But whether the issue is large or small, do yourself a favor and don't complain too often, or people might stop taking you seriously.