It's a number that the computer expects incomming data packets to specify for specific destinations. It lets computer know which running program to send the packet to. There's many programs that simultaneously run on a computer. It is not omniscient, it needs a number to know where to send the packet further.

When a country receives a phone call, they need a specific phone number to know which person to connect. Phone number in this case is like a port. And country like a computer.