Drop is a basic, activated ability with the following text: Once on your turn, if this unit hasn't interacted with the realm, it may drop any number of artifacts it's carrying. Each unit can use its Drop ability once per turn. Since Drop is an activated ability, you can only use it during the main phase on your turn while nothing is on the storyline. For example, you can't use the Drop ability while in the middle of the Move and Attack ability. If the unit has the ability to carry other units (e.g. War Horse), then its Drop basic ability is modified from 'artifacts' to 'artifacts and/or units.' See the 'Pick Up' and 'Interact' codex entries for more details.