The word Unwrap a 3D object means transform it in to 2D. In order to apply a texture to it. There always a zillion of tutorials on the subject, but picked up the tutorial that is more objective and explain what really is unwrapping a 3D model, in a quick and objective way. Following the series logic on, objectivity over complexity.
Textures are a important part of the object because not all things are done in the shape or in the physical 3D object. Some details look like a 3D shape but are a texture. The texture is important, so that one can add extra details with out making the 3D model to complex. The 3D shape can always have more triangles and become bigger in size. By the other side, adding more detail to textures it uses less triangles and the 3D model become smaller, more usable for games.
One often see : low poly and high poly resolutions; High poly is a resolution for movies, animations and scene creation; for games most of the time they are build in a Low poly format. Unless you are creating some kind of AAA title, which is not the case. A budge for that kind of game can be around millions. Low poly or medium poly count is enough for your game resolution.