Ein Polizist hält das Auto an.
Firstly, I didn't understand why there is "an".
My translation to this sentence is
A Policeman stops his car.
After I read this question below,
What is the meaning of 'an' in context “Tief in mir ist noch ein Feuer an”
I guess that "an" means "at somewhere". Like,
A Policeman stops his car on the road.
Is it right?