Things come into focus when the rays of light meet up at the same place at the back of your eye. Since all of these rays come from different directions, you need the lens in your eye to bend them to meet properly (or glasses/contacts that do the same).

If you look through a small hole directly in front of your eye, you're eliminating all of the rays of light except those that go straight into your eye and hit the back of your eye at the same point.