I followed this video to make my Home Asssistant accessible from everywhere, and yeah, port 8123 is fully clear to me, I've done it, it works great.

But he also talks about port 443, and when he sets it up in the router instead of choosing 443 on both WAN and LAN he goes for 443 to 8123. Why? And which one is the WAN and the LAN one? His router config has completely different names to mine, and as you can tell I am quite a noob at this.