Inscription is the third type of "socket" enhancement for your gear, and the final Cube unlock at level 25. It shares the RNG nature of Decorations and Engravings, but truly takes it to a whole new level.
Only Arcana Grade gear and above will have Inscription Slots, so it won't be relevant until you've progressed through a good chunk of the game (unless you get a lucky drop or hit up the market).

Inscription Materials offer a large range of stat boosts, making them universally applicable, yet extremely unreliable. That said, the numbers do look quite strong comparatively.

Finally, given the quality of gear that's required, I suppose Inscriptions can be thought of as more of a min-max addition. While it may take multiple attempts to get the right roll, the result will be worth it!