Hmm...for someone who has been in AQ for 3 years...
Anyway, substrate generally refers to the layer of soil, gravel or sand at the bottom of your tank. It can come in different color, and types (e.g. round and non rounded gravel), size, and properties (neutral, or hardern/soften water). Some are man made whilst others are naturally occuring.
Hope that helps.
Cheers,
Bookmarks