What does "incoming" and "outgoing" traffic mean?
Solution 1:
"Incoming" and "outgoing" are from the perspective of the machine in question.
"Incoming" refers to packets which originate elsewhere and arrive at the machine, while "outgoing" refers to packets which originate at the machine and arrive elsewhere.
If you refer to your web server, it mostly accepts incoming connections to its web service, and only occasionally (or maybe never) makes outgoing connections.
If you refer to your web client, it mostly makes outgoing connections to other services, and only occasionally (or maybe never) accepts incoming connections.
Clear as mud now?
Solution 2:
In your case you only have to let incoming requests to port 80.
When a connection is established, the firewall will automatically let packets out back to the client's port. You don't need to create rules for that because the firewall knows.
Solution 3:
Without any context as to what the particular text you read means when they refer to "outgoing web service" traffic, I'll take the simplest approach in my answer:
You have a firewall at the ingress/egress of your network.
The firewall comes in a fully locked down state and allows NO inbound or outbound traffic.
In order for your internal clients to browse external web sites you need to configure an "outbound web service" rule that allows them to connect to said external web sites.
In the simplest terms the rule would read something like this:
ANY internal host to ANY external host where the destination = TCP Port 80 then ALLOW.