The Trip Mine is a special type of explosive that goes off only when a player interacts with it.
There are two ways to arm this explosive, the first is to just press Interact Key (Default F/Enter) while holding it, you will drop the item and see a message to confirm it is armed. Now, whenever this item is picked up, it will explode.
The other way to arm this explosive is while it is inside any container , such as a vehicle trunk, a safebox or even a bag, click the item in the list to open it's options menu and click "Arm Trip Mine". Now, whenever that container is opened, it will explode.
For more information, visit the Explosives page.