It's a string of characters that identifies a resource on the internet. This could be:
* A web page: like `https://www.google.com`
* An image: like `https://example.com/image.jpg`
* A file: like `ftp://ftp.example.com/file.txt`
* A service: like `mailto:[email protected]`
Essentially, any resource that can be accessed online can be identified by a URI.
URL (Uniform Resource Locator) is a type of URI that specifies the exact location of a resource on the internet. So, every URL is a URI, but not every URI is a URL.