One point perspective is a method that shows how things appear to get smaller as they get further away, converging towards a single 'vanishing point' on the horizon line.

A vanishing point is "the point at which parallel lines appear to converge". The reason we perceive a vanishing point when looking at those two lines is because of that simple principle: objects appear smaller the further away they become. So it follows that the distance between two parallel lines will also become smaller as the lines recede into the distance, which is what makes them appear to converge.

Also called "single-point perspective," this very simple idea can be a very powerful tool in simulating three dimensions in any two dimensional work of art, whether it's a painting or a photograph.