You can tell them to imagine the internet cable as a highway, and the data packets running to and fro from your computer as little cars traveling along the highway. All we need to focus on is the cars coming towards your computer from the highway.
Now, your computer ports are like stops and on-ramps along the highway. Each car has a specific destination. Trouble is, they don't know where it is, so they'll all be stopping at each of the 60,000+ stops until they get to the right one. The highway's moving quite slow.
Port forwarding provides the data packets (cars in this brown townogy) with highway signs pointing to the locations of specific ports. "Opening" port, say, 100, and the signs will show arrows pointing to the location of exit 100. The cars will know where it is, and they don't have to stop at every exit and can simply cruise to their destination. In the electronic world, this sign only really increases the speed by a few milliseconds but when doing something rather network-heavy (like online gaming) it adds up.
Bam, the highway's moving faster.
But that's all port forwarding really does: Adds highway signs. It doesn't remove any security features or the firewall at exit 100. Plus, the other cars that aren't going to exit 100 simply will ignore the signs and will continue on their trial-and-error journey of the 60,000 different ports. The hacker cars probably aren't going to any of the exits you signed, and even if they did make it to them eventually they've still got to go through the security checkpoint anyway.
