Julia, capillary mat is just some type of fabric under the flat (with holes in flat) that drapes over into a well of water. The fabric absorbs the water and the plants have water available to the roots. Check this out, but you can make your own:
http://www.gardeners.com/APS-P...
When my DH was in charge he thought the plants looked wet enough, he didn't touch the soil so mine dried out instead of drowning!
Edited to correct spelling