What do you mean by fill up?

By fill up, we mean to make something completely full to its capacity or to become completely full, emphasizing reaching the top or maximum limit. For example, fill up a gas tank means pump fuel until tank cannot hold more. Fill up a glass means pour liquid until glass is full. Fill up a suitcase means pack items until no space remains. As intransitive, the room filled up means it became full of people. Fill up can also mean to eat until full, as in fill up on snacks before dinner, or to cause eyes to fill up with tears. In everyday speech, fill up is used for liquids, containers, and spaces, and it conveys completeness, not just partially filling. It is similar to top up but stronger, suggesting going from not full to fully full. In informal usage, fill up can mean to eat a large amount. For example, fill up the car, fill up on water. So when someone says fill up, they mean add enough to make something reach full capacity or become full itself.