The subfloor in any structure is the rough floor under your finish floor. This can be a concrete slab or any material laid on top of [joists]. The most common material to use is [OSB], but [Durarock] is also a common subfloor used under tile. If your floor has creaks or soft spots, it usually has to do with loose or damaged subfloor.
Links leading to external websites (mainly amazon) are affiliate links. This means I receive a small commission if you purchase anything, after following my link. This comes at no expense to you. If you would like to read more here's our full Disclosure.