what is Substrate?

[pbc]

testing the "there are no stupid questions"

I see the word used a lot, and am getting ready to install tile on a newly repaired, leveled floor

[thedcdude]

The "substrate" is what you use directly beneath your tile. Usually some type of cement backer board.

[pushkins]

Yep, you need to add 1/4" backer board to that floor before you tile, make sure you trowel some mortar or tile glue onto the ply then lay your backer board and screw it down ( some say you can nail it down with roofing nails if you use mortar in between.
I never do, I always screw it down.