For this article, understanding network address translation is required.
When making use of a usual SoHo router (like most ISPs give to their customers) and using IPv4 addresses, hosting services behind the router requires to bypass the routers NAT. By default, this is not possible. Port forwarding is kind of punching holes into that mechanism. You configure your router to always relay packages arriving at a certain port on the internet side to be forwarded to another computer on the local network, to a specific port. This can also be a list of ports being forwarded to a list of other ports. There is a distinction between TCP and UDP packages, too. Don't mix them up. The outside port and the inside port do not necessarily have to be the same. So the public port 8080 may in fact point to a local computer's port 80.
This mechanism is also sometimes named simply as forwarding or unblocking ports on the router, or (falsely) unblocking this port on the routers firewall. (Most SoHo routers don't have any firewall.)
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