What does "fill up" mean?

Fill up means to make something completely full or to become completely full, emphasizing reaching maximum capacity. As transitive verb, fill up means to add substance until container or space has no more room, such as fill up a gas tank, fill up a glass with water, or fill up a suitcase with clothes. It suggests topping off to full. As intransitive verb, it means to become full, as in the parking lot filled up quickly, meaning it became full. Fill up is often used for liquids, gases, and spaces, but also figuratively for feeling full after eating, as in I filled up on bread, or for emotions, as in eyes filling up with tears. It is slightly more emphatic than just fill, implying completeness. Phrasal verb fill up is common in everyday speech, like fill up the car with gas, fill up on snacks, or theater filled up. In informal contexts, fill up can also mean eat a lot, as in fill up before trip. It is similar to top up but suggests fuller extent. Overall, fill up conveys action of making or becoming completely full to capacity.