Cemuhook, also known as Cemu hook, is a plugin created by Rajkosto with help from Exzap, which started as a solution to the lack of H.264 video support. This plugin also featured the first use of the GX2SetGPUFence function, Cemu's first hack. The main uses of it are to support assembly patching with "graphic" packs (with every pack that includes a patches.txt) and until Cemu 1.18.0 it was the only way to have accurate motion controls in Cemu.
Download and extract the ZIP file on Cemuhook's site that correlates with your Cemu version where your Cemu installation is.