Stuff is coming into your network all the time. Your router and PC examine the communications and deny whatever looks wrong. It's likely you don't even realize that there's stuff sniffing around your network connection all the time, but your security is blocking it. That's why it's very important to keep you computer's security software updated all the time.
If you don't let anything into your network, you won't have internet. You have to let some stuff it. Port forwarding, which is going to be required regardless of which option you choose, must be activated. It gives the router a "route" from outside your network to inside your network, but ONLY to the IP you say. It really isn't very hard to understand. You already do it all the time and don't know it. It's called a Post Office. You give them your address so they can deliver stuff from outside your city to you. It's how you get your orders from Amazon and Liveaquaria

A router just goes one step further and tells the mail which room in your house to go, but no where else.
I'm also a network engineer and managed thousands of computers on a complex network that spans a city. Once you learn the basics, it just repeats on a larger scales with networks.