When an object is close to me, you can use a ruler to measure the distance. But, it is difficult to measure when the object is away from me. In this case, how can you measure the distance?
Humans have two eyes. The world with the left eye and the world with the right eye are slightly different. If the object is very close to me, the world seen with the left eye and the world seen with the right eye look very different. Parallax is an angle that varies depending on where you are observing. Parallax increases as the distance of the object gets closer.