1. add what is needed to make something complete
fill a gap in knowledge/understanding
To ‘fill a gap’ means to add what is required in order to fulfill a need or make something whole or complete. A ‘gap’ is a void or empty space. You can ‘fill a gap in knowledge’, ‘fill a gap in a collection’ ‘fill a gap in staffing’ etc. You will hear this used more often in professional contexts or formal situations.