^^imagine ur modem to be a big big wall with some holes here and there to go on the opposite side... then these ports are those holes
... they are given numbers... identification of valid packets are done and then only they are transferred thru ports to the vast internet through modem... similar is the thing for incoming packets (packet=small part of information). If this is the situation then port forwarding merely means 'makin' a hole yourself for speedy data transfer. i m knowingly not using 'technical' words to make u exactly understand wats actually goin on