UV mapping is the process of projecting a 2D image onto a 3D model’s surface. By unwrapping the models surface onto a 2D space, you will produce UVs. UVs act as marker points that control which points (pixels) on the texture correspond to which points (vertices) on the mesh.
This process allows you to correctly place a texture on a model without any deformations (if done right). Below you can see an example of the incorrect and the correct texture mapping on a plain sphere.
To get a better idea of what UVs are, here is an example of the complex model with its unwrapped surface (UVs) on the left side.
Mapping from primitive shapes
The easiest way to map a texture to the model is to do it by mapping from primitive shapes – planar, box, cylindrical and spherical. You can do this by choosing a mapping from the right panel of the UI, or rotate the mapping options with the shortcut CTRL + SHIFT + U. After choosing a mapping, your UVs are selected and you are able to move, scale or rotate your textures through gizmo operations.