A handful is just a little of something, or the amount you can hold in your hand. It can be a literal handful, like a handful of popcorn, or it can mean just a few, like a handful of trucks.
You might find yourself eating handfuls of raspberries when you're supposed to be picking them to make jam. Another meaning of handful is "just a few," as in "There are only a handful of dogs at the dog park this morning." And a third way to use this word is to mean "incredibly difficult to handle." If your mom says, "You were a handful when you were a toddler," she means you were not easy.