On a internet level (not only web), the term resource is used in a general sense for whatever might be identified by a URI (ie a URL for the web)
They are application resources and should not be confound with hardware/machine resources
In a process context and in general, the term resource means what is limited. See resource.
A document (ie served by a server):
A file (served by a What is a File System (filesystem)?)
A service:
Or Objects:
A resource might not be singular in nature. A resource might be:
A resource can be:
see Browser - Fetching Resources (Request/Response)
See What is the Mime? known also as Media Type
Resource can also be represented as a graph object by adding metadata