TDLib or Three's Derma Lib is a derma library created by Threebow allowing you to create beautiful Garry's Mod UI quickly and without hassle. No more messing with paint functions.
View the documentation here
Move the file into the lua/autorun/client of your addon. Do not edit the file in case there are multiple copies of it on the server from other addons, to avoid confliction. If you feel a modification is neccesary, open a pull request.