Matting is to composite several elements one after the other onto the same film. As opposed to double exposure, here each part of the frame is exposed only once. While capturing one part of the image the other parts of the film, that are not to be exposed, have to be blocked/ masked out.
So mattes/ masks are the supplementary plates that block out certain portions of the film. Mattes can be of two types :
Static mattes – The shape of the mask does not change from frame to frame
Traveling mattes – The shape of the mask changes with the action.
Here is an example of a static matte where the window pane is masked out and later filled in with the footage of a train passing along: