A molehill is a small pile of earth pushed up by a mole, the little animal that digs tunnels under grass and fields. The pile is just loose dirt, often on a lawn, and gardeners get cross about it. In real life that is all it is. Though actually, English speakers almost never discuss the pile itself. They use the old saying make a mountain out of a molehill: treat a small problem as if it were huge. From that saying there is also a rare verb, to molehill, but you can live a long time without meeting it. The word is ordinary and a bit old-fashioned in the idiom; younger people still understand the saying even if they have never seen a real molehill. This mole is the animal, not the chemistry mole and not a spy.