The conventional second HTTP port. Used by application servers behind a reverse proxy, by development servers, and by admin interfaces on appliances.
Frequently where a forgotten admin panel or an unprotected staging app lives. Worth checking what is actually listening on 8080 on your own hosts.
Find out what is actually listening before deciding anything. It is commonly a forgotten admin interface or a staging application that was never meant to be public.
The port scanner will tell you whether 8080 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.