Ao is the contraction of 'a' and 'o' i.e. "to the" or "at the." So, it's basically saying, "they stop and look at the surroundings" or, less literally, "they stop and look around."
But why didn't they use it with "homem"? Nós olhamos o homem (I think that's how it was written)
Parar doesn't have a voce/ele/ela imperative? (it's missing in the conjugate table)