Fiberglass and resin are two parts of an equation
Fiberglass is normaly composed of glass fibers, though it can be mixes of different materials. It's very flexible but has no rigidity, but it is resistant to pulling forces along the length of the fibers
Resin is normaly a two-part epoxy it hardens when mixed, but it is not flexible and is rather easily broken
When you combine the two, the resin gives the fibers rigidity while the fibers allow the resin to resist bending & breakage
fiberglass cloth has fibers arrayed in a uniform crossing pattern, a cloth weave; matting is not orderly, and is often used as a strengthening filler
this combination is sometimes called a matrix and is something that actually exists, no matter what pill you choose