Apply or Applying is an action performed by pressing the appropriate button when available.
Applying is an action that deals with medicines. This action allows the hominid to use the medicinal item in a manner other than consuming the item such as eating or drinking.
Some items can only be applied such as Kapok Fiber.
Some items have different medicinal properties when applied versus being eaten. Honey can be applied to heal Bleeding but when eaten it instead heals Venom Poisoning.
Mechanics
The item to be applied must be in the hand of your hominid for this action to appear. Sometimes you may have to switch hands for the action prompt to appear.
Pressing the apply button will cause the hominid to use the item on their body. There is a small animation of the apply spreading or dabbing the item onto the body. Once the item is applied it may or may not be used up. Check to see if you still have the item in the hand of your hominid and you may be able to use the item again.