What are mats?

Please forgive my ignorance but I just found this site a few weeks ago. I was making the cane condos long before I read about them here, but I am reading about people using mats in shallow water and I am curious. What do they look like and how do you make one? I have some shallow water spots that really attract crappie, but I would like to have a few structures in that area of the lake that all the other crappie fishermen don't know about. The exposed brush tops get hit hard.

http://www.actionfishingtrips.com/habitat.htm

Look about halfway down the page for info on mats.

Thanks Woohdog, good info!

thanks for the link, good info.