Trinkets is built using Fabric, it will not work on Forge.
Trinkets adds modular accessory system to Minecraft. Slot groups are collections of slots for a certain body part. By default there are 6 slot groups (head, chest, legs, feet, hand, offhand) that can have slots added to them, but more groups can be added if desired. By itself, Trinkets only implements one slot (chest:cape) and reimplements the elytra to use this slot, mods are responsible for adding any slots they plan on using.
Documentation for how to use Trinkets in your mod is available on the Github source for this project.
Feel free to use Trinkets in your packs.