NylonX is simply a modified version of the nylon fiber manufactured by MatterHackers for 3D printing. It belongs to their PRO Series of 3D printer filaments.
This particular filament is created by reinforcing its original material (nylon) with micro-carbon fibers. As a result, the produced threads become stronger which is necessary for building sturdy models with 3D printing.
Along with better stiffness, the NylonX filaments also offer higher resistance and tensile strength for the end product. It essentially brings you the best of both worlds by combining the rigidity of carbon fiber and the durability of nylon.