When you grumble, you complain about something clearly but quietly, in a low mutter. Some people grumble a hundred times a day, about everything from the weather to the traffic to their bad luck at bowling.
The verb grumble is a great example of onomatopoeia, a word that sounds just like what it means. When you say "grumble" out loud, you're practically already grumbling about something. The origin of grumble is the Middle French word grommeler, or "mutter through the teeth." Just think about the things in life that make you want to mutter through your teeth and you'll probably start to grumble.