its like valves on a real engine.

this picture is a pic of the Sleave of the Novarossi 528xr

each one of those "holes" in the sleave is a port. a mixture of air and fuel enter threw these into the combustion chamber to be well... combusted, then the gasses are exited threw ports too (called exhaust ports).

you really have to under stand how a 2stroke engien works to really get what ports are.