To overfill a container is to put too much of something into it — a cup, a box, a suitcase, a petrol tank — so it is too full, and often spills or will not close. You overfill the bag, the glass, the bottle. It is a practical, slightly warning word: don’t overfill it. Though actually, students often mix it with overflow. Overflow is what the liquid does when it comes out over the top; overfill is what you did to the container. In workshop English you also meet overfill as a noun (overfill protection on a tank), but school English mostly wants the verb.