What does it mean by dimensions with matting?

What does it mean by dimensions with matting?
Assuming you're talking about a picture frame, or a framed photo, poster, or art work, mattes are those wide solid-color pieces of thick paper or board that are put in the frame on top of the artwork, like an outline.

A picture frame's dimensions with matting would mean that an 8 x 10 frame with a matte inside may only be able to display a 5 x 7 photo, because that's how big the opening in the matte is.

If you're trying to buy a poster, photo, or art work, dimensions with matting means that the poster, photo, or art work will include a matte, which may be bigger than the poster, photo, or art work itself. (Like above, if you were buying a 5 x 7 photo, with matting it may be 8 x 10.)

Mattes can be very wide, or very skinny, so the dimensions with matting may not actually give you a very good idea of what size the actual frame or poster, photo, or art work really is. If you're concerned, ask about the frame size (if buying a frame), or the size of the actual piece (if you're buying a poster, photo, or art work.)

Remember, if you're buying something you're going to frame, the frame itself will make the dimensions even bigger, so measure your available space carefully!