To explore the concept of perspective and horizon, consider observing a table from different heights. When seated, your view of the table’s surface is limited. Standing up offers a broader view of the tabletop. Crouching, however, not only provides a different angle of the same surface but reveals a portion of the table’s underside. So, we can see how our viewpoint can significantly alter our perception of a single object.
The horizon can be understood as the observer’s eye level, serving as a fundamental element in perspective drawing. It is the point where all perspective lines converge, creating a sense of depth and dimension in the representation of space.
You can start observing by doing the exercise given at Eye level, don’t get disappointed if you don’t get them all correct.
Other material that is very helpful, and I would advice studying: