v.1. To become full: The bucket filled up with rain. The train filled up quickly, so we had to stand in the aisle.
2. To cause something to become full: I filled up the sink with soapy water. We inflated the pool and filled it up with water.
3. To fill some vehicle with fuel: I said to the mechanic, "Fill it up!" You'd better fill up the car soon. If we don't fill up now, we could run out of gas.