A resource on the web is everything that could be uniquely addressable/identifiable. A resource is a fundamental element on the web.
Resources are things I want to interact with on the web; It could be web pages, images, files, audios, videos, static resources (CSS, js…), an e-mail, information from various data sources like databases, SharePoint servers, and web services, location on google map and so on.
Earlier, a resource was static file or document. Now it has gradually evolved and nearly everything that can be obtained via the Internet is termed as a resource.
URI (rather URLs) are used to find/identify/locate a resource on the web and MIME type (media types, a two-part identifier for file formats and format contents transmitted on the Internet.) to specify the representation of the resources.
The web and HTTP are both all about resources.
Everything was designed to just make things work. So that different systems (Unix like OS, Linux, Microsoft Windows, Mac OS and so on), different platforms can easily work with each other.
Example of resources:
https://en.wikipedia.org/wiki/Main_Page — Web page
https://en.wikipedia.org//static/images/project-logos/enwiki-2x.png — an Image file
https://upload.wikimedia.org/wikipedia/en/3/3d/Sample_of_Daft_Punk%27s_Da_Funk.ogg — Audio file
https://www.google.co.in/maps/place/Pune — Location on map
https://www.onlineshopee/items/ — Collection of items
https://www.onlineshopee/items/123e4567-e89b-12d3-a456-426655440000 — Specific item
IMO, Resources on the web are as important as Objects in object-oriented programming.
Once you understand this terminology, everything (HTTP, REST, Web services and so on) based on this concept becomes easy to understand.