When a web server reports it is listening on 0.0.0.0:80 it is accepting connections on every network interface. In a routing table, 0.0.0.0/0 is the default route — "everything not matched elsewhere". A device that shows 0.0.0.0 as its IP has not received an address from DHCP.
Ad-blocking hosts files often map domains to 0.0.0.0 instead of 127.0.0.1 because the connection fails immediately rather than waiting for a local service to refuse it.