A floating mat is when the window mat is cut to an opening larger than the print, this leaves the total print and some of the board that the print is mounted to visible. The advantage of this is that the window mat never contacts the print surface, which eliminates the possibility of the window mat leaving an impression on the print surface.
Also mat board will absorb acid and pollution from the air and even buffered board may eventually turn acidic. If you use the alternative to a floating mat, an overmat, the acid may be transferred to the print due to it's direct contact with the print surface. The disadvantage is that you have to mount a floating mat print more precisely and you need to have clean edges on the print.