What does the projection of solid geometry in high school mathematics mean? I can't understand it directly.

you should have learned it in physics class in senior high school, right? Most of the light sources in real life are a surface, such as the sun, candles, flashlights, etc. However, the shadow formed by this light source after illuminating the object has virtual light, that is, the cage around the shadow is unclear and there is no obvious boundary. The projection of the object on a fixed surface under another light source varies with the position of the object between the light source and the projection surface, and the size of the image formed on the projection surface is also different. In order to solve these two problems, people put forward an imaginary light source. The light of this light source shines on the object in parallel, which solves the above two problems. One is that there is no virtual light, and the other is that the shadow size formed by the object is constant no matter where it is placed between the light source and the projection surface, which is very convenient to apply in many disciplines.

Projection means that the object forms a shadow on the projection surface under the parallel light source, and this shadow is called projection.