The Compound system, accessible by pressing SHIFT + P, is an enhancement system designed to help boost your character’s base stats by adding unused accessories and fusing them with Runes. Compound allows for a total of 6 different items to be registered. However, choose wisely; accessories that are used in the Compound system cannot be retrieved.
To start, if you hover your cursor over certain accessories, you'll find a section for the system called Power Potential. Each accessory has a different amount of Power Potential based on its tier. The higher the Power Potential, the higher stats you'll receive after adding them to the Compound system.
In addition to these accessories, you may also acquire Hongmoon accessories, either from salvaging certain items or from events such as Soul Boost. These come in sets with Power Potential just like the ones in the table above.
Once accessories have been placed in the Compound system, all that's left is to acquire Etching Stones to engrave stat bonuses onto Runes.
Etching Stones are used to activate Runes in the Compound window. Press the + button to open a list of available Etching Stones to add in. You will get different amounts of stats depending on the tier of the Etching Stone. You will also get different attributes depending on the Rune. For a list of the different types of Runes as well as their stats, please read through the table.
Typically, users find that the Runes with the best stats for PvE scenarios are Tenacity, Torrent, Execution, Aggression, Onslaught, and Prayer.
Thanks to the contributors in the Alice Kr Stuff Discord Server (see Useful Links for more info), several detailed sheets are available with tons of data surrounding the Compound system.