Web server behind a router. How to access the server using external/public IP address?

I'm new to the server stuff and I'm trying to figure out how to forward an http request through external public IP e. g. 10.20.30.40 to a local server machine address i. e. 192.168.x.x. I tried to google this question but couldn't find any clear-cut solutions. Some people say you have to use port forwarding but I'm not sure what that means exactly. This may seem like a fairly easy task but I'm really without a clue here. Obviously, if I type my public IP address in the browser I get to my router.

Host machine OS: OpenSUSE Tumbleweed & LAMP. Router: TP-Link TL-WR841N.