areppim: information, pure and simple
You want to calculate the probability (Poisson Probability) of a given number of occurrences of an event (e.g. customers entering the shop, defectives in a box of parts or in a fabric roll, cars arriving at a tollgate, calls arriving at the switchboard) over a continuum (e.g. a specific time interval, length, volume, area or number of similar items). Just enter the requested parameters and you'll have an immediate answer.
Please walk through some Poisson Probability examples.