Port forwarding is technique by which you can access your local sever over internet.
All http traffic of your system is handled by router i.e your ISP. There are two IP address assigned to each system in LAN which has access to Internet i.e Local IP and global IP.
There are few configurations by which your router allows you to access other system if you both are connected to same LAN by using your Local IP. Your local server traffic is blocked by your router so it is not accessible outside LAN.
When you want to access your system through internet port forwarding comes in picture.
Port forwarding allows you to access your localhost by managing some configuration over your
Server configuration files and ISP configuration files. If your internet port i.e Port 80 is preoccupied you can change your server outgoing port to manage traffic.
Port Forwarding has security limitations so it is generally not suggested for private data.
You can use a static global IP for more secure use of this technique.