Two of the criteria for a counting process to be Poisson are the following: (a) $\displaystyle P \{N(h)= 1 \} = \lambda h + o(h) $ and (b) $\displaystyle P \{N(h) \geq 2 \} = o(h) $.

So this is saying that the probability that the count exceeds $\displaystyle 2 $ is essentially $\displaystyle 0 $?