UV is an acronym referring to U and V coordinates. It can mean different things in different contexts:
In the context of 3D art, U and V represent the mapping coordinates for a 2D texture to lay across a 3-dimensional mesh. Because of this 2D to 3D mapping, a different coordinate system is used. U and V are roughly analogous to X and Y coordinates (and are the letters immediately preceding X and Y in the alphabet). Within the context of HeroEngine, an artist can also create animated UVs, by setting U and V coordinates to scroll at separate speeds, to make it look like an applied texture is animating on the mesh surface. This is useful for creating such things as waterfall effects, moving molten lava, or moving conveyor belts without the need to rely on mesh animation.
For example, if the artist animates just the U coordinate of a 2D texture, the texture will appear to slide along the local X-axis of the mesh.