Unencrypted web traffic, and the original port of the web. It is still the first thing most browsers and tools try when given a bare hostname, which is why it remains in use even on sites that serve nothing over it — a request to port 80 is answered with a redirect to 443.
Anything sent over port 80 is readable and modifiable in transit. Its remaining legitimate job is to redirect to 443 and to answer certificate validation challenges.
Worth knowing. Redirect to HTTPS with a 301 and add HSTS so browsers stop using port 80 for your domain at all.
Open 80 is normal. The question is what it does: it should redirect to HTTPS rather than serve content directly. If it serves the site, traffic is readable in transit, so add the redirect and HSTS.
The port scanner will tell you whether 80 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.