For questions related to the UV editor and it's tools.

The UV Editor is used to map 2D assets like images/textures onto 3D objects and edit what are called UVs.

uv editor image_

The most flexible way of mapping a 2D texture over a 3D object is a process called “UV mapping”. In this process, you take your three-dimensional (X, Y & Z) mesh and unwrap it to a flat two-dimensional (X & Y … or rather, as we shall soon see, “U & V”) image. Colors in the image are thus mapped to your mesh, and show up as the color of the faces of the mesh. Use UV texturing to provide realism to your objects that procedural materials and textures cannot do, and better details than Vertex Painting can provide.

See the Blender Manual for more information.