It means binding the model's surface's certain UV co-ordinate to texture's XY-coordinate, so that the rendering engine knows to show correct part of the texture on the corresponding part of the model.
UV co-ordinates of a model are simply a system of identifying each location of the model's surface individually, like XY-co-ordinates mark the locations on a plane, XYZ-co-ordinates locations on space, and XYZT in time and space (or four space dimensions, whichever you prefer...).
AFAIK it is perhaps the slowest process on making a model presentable, because you practically need to move each face individually to a suitable place on the UV map... and the more complex mesh, the more faces there are.
Would you like to know more?
