The Radial Equipment Menu is a unique equipment mechanic.
Certain items found or made in the Betweenlands (mainly rings or amulets) will have the 'equippable' tooltip on them, meaning they can be equipped as part of the radial menu. The menu can be opened with the F key by default when one of these equippable items is in the inventory and/or already equipped. A circular menu will appear with buttons that display any equippable items not currently equipped that CAN be equipped at the time (with a green tab saying 'equip') as well as any equippable items currently equipped (with a red tab saying 'unequip').
Left-clicking green tab buttons will equip the displayed equippable onto the player, while left-clicking red tab buttons will unequip the displayed equippable from the player into their inventory again. Note that only one of any ring type or pouch can ever be equipped at a time, while the number of amulets that can be equipped at a time varies depending on how many times an Amulet Slot has been used before death. While equipped, items will appear in a small space to the right of the hotbar.
Here is a list of all currently available equippables in the Betweenlands that use the radial menu: