Sir Tim Berners Lee has been quoted as saying "The web is a mapping from URI's onto meaning".
URIs are the global identifiers that enable resources to be identified on the web.
URIs are fundamental to the four principles of Linked Data:
1. Use URIs as names for things.
2. Use HTTP (Web locatable) URIs so people can look up those names.
3. When someone looks up a URI, provide useful information.
4. Include links to other URIs, so they can discover more things.