Break this word down, and it will be easy to remember. “Sub” means beneath or under, and “stratum” or “strate” means layer. So, naturally, substrate means a foundation or sub-layer that provides a base for something else. It’s an underlying support, and most commonly it is used when talking about surface materials: flooring, wall, counters, ceilings, etc. Bonus info: Just as a building is only as good as the foundation it’s built upon, finish materials are only as good as the condition of their substrate. For example: You can purchase and install the most gorgeously expensive handmade wallpaper in the world, but if the substrate is warped, pock-marked or otherwise marred in some way, the finished product will end up looking as though you bought it at your nearest discount store.