Disclaimer: I am not a professional network engineer, this is just what i found out after researching some iptables terminology.
the lan => wan is perhaps a bit misleading. lan is the zone, or which side of the router this firewall rule is in reference to. wan is another zone, the arrow shows where packets of type Forward are ending up.
Input means packets originating from another device within this zone with the router as the destination.
Output is a packet from the router to another device in the zone.
Forward is a packet originating from one zone with a destination in another zone.
When forward on the wan interface is set to reject, it essentially means no device from outside may initiate a connection. However, they may respond to already opened connection.