The nice things about the human mind is that when talk about things humans take into account representations of this things and not the things necessary itself.
Humans share common concepts. On the basics of these common concepts humans are able to communicate, communicate information and to understand information. We need to identify things we are talking about. We need to find representations also in the Semantic Web.
If talking about a Object in the mind of human the Concept of this Object and all its all the general properties occur, i.e. all the properties common to all objects associated with a Concept. If dealing with things, human take into account the representation of these things and not the things itself.
Uniform means we can identify different types of resources and we have an identifier that is constructed according to a uniform schema. This means we can identify anything, any resource we can think of, can be identified via a uniform resource identifier. Of course this identifier must be used to distinguish one resource from another resource in a unique way.
What you can identify with and URI is a resource of any kind. The concept of the URI is established in the web via the URL, the Uniform Resource Locater, which is an address of an document in the web or with persistent resource names or persistent URLs. Books are idenfid via ISB and ISS number. Things the web can also generally be identified with a DOI.