Imagine that everything becomes smaller and smaller as you get farther and farther away.

At infinite distances, everything becomes so small that it becomes one small dot. This one small dot contains a small chunk of the entire universe.

That's the concept behind the one point perspective.

Everything that's properly lined up at 90 degree angle, parallel, converge into one small dot in the distance (it's called the vanishing point, and the vanishing point usually is a point on the horizon line if there is a horizon).

So, anything that is parallel to each other and at 90 degree to the viewer does not align parallel to each other. This is the contradiction of perspective, that parallel objects converge at what is seemingly non-parallel angles, and only non-parallel objects may be situated at certain angles that make the objects appear parallel when they are not in reality.

For two point perspective, two perspectival lines go from you the viewer to the horizon line at different angles, and they connect at a point below the horizon line, usually at 90 degree angle (although there can be distortions or they can be at different angles than 90 degrees.).

Just draw two vanishing points far apart from each other on the horizon line and draw one of the perspectival lines towards you and connect it to a second perspectival line where they should meet.