When you enlarge something, you make it bigger. If you build a huge addition on the side of your house, you enlarge it.
You can enlarge a sand castle by dumping another bucket of sand on it, or enlarge your hairdo by curling and teasing it. When a photographer enlarges a photograph, she blows it up, or prints a much larger copy of it, and you can also enlarge an idea, or someone's understanding of it, by clarifying or explaining it. The Old French root of enlarge is enlargier, "to widen, increase, or make larger."