In semantics, the instrument is the thing or means used by the agent to effect an action. In English it is often expressed in a prepositional phrase using the preposition with:
We dug holes for the plants with a spade.
She opened the door with her left hand.
He cut the cake with a large knife.
We measured the temperature with a thermometer.
However, it can also occur in other constructions - eg as the subject or object of the verb use:
A thermometer was used to measure the temperature.
She used her left hand to open the door.