Player API 1.13 is a type of Library.Core/API which Mod Developer of many famous mods such as Aether, Smart Moving Mod, Flan’s Mod, Galacticraft Mod uses to build mods or add new content to the Minecraft modding community.
With Player API, you can access to client, server, and bukkit classes and simultaneously reduces incompatibility between mods. Many popular mod was created basing on Player API which makes Minecraft community develop much more. However, until now most of mods are not depended in this mod anymore, just some mods from author of Player API still use this mod such as Square Moving or Smart Moving. If you are player who usually implement many mods at one time, this mod is surely indispensable.