This explains it better than I can:

http://www.isogg.org/wiki/Autosomal_DNA_statistics

By the way, just to make absolutely sure that the match is maternal, you'll need to get you and your mother's results up on GedMatch. If the match ever responds to you and indicates an interest in exploring your connection further, you can suggest he also sends his results to GedMatch.

Until you triangulate your Ancestry matches and look at them through a chromosome browser, you cannot be 100% certain what your relationship is.