In one-point perspective, one "face" or plane or side of a cube or cube-like form is parallel to the picture plane, facing the observer directly. The left and right sides as well as the top and the bottom of the cube all converge on a single vanishing point located on the horizon line/eye level. Edges perpendicular to the picture plane converge on a single vanishing point, while all vertical edges are parallel to the picture plane and remain vertical with no evidence of convergence.
The easiest way to understand one-point perspective is to envision converging railroad tracks or a sidewalk retreating in the distance.
For one-point perspective to function correctly, the observer must do several things. Position yourself parallel to the subject (i.e. buildings or walls). Maintain your position/station point. Maintain your line of vision.
You must remain in a fixed position throughout the drawing because ever time you move your head, even just your eyes, up and down or side to side you change your point of view and the drawing loses consistency and therefore its credibility.