What does the XML syntax with a colon mean?

What does:

<rd:...>  </rd:.....>

in XML format of RDL file mean and the purpose of it? Anyone please explain it to me. I just started learning this.

The XML syntax, rd:, is a namespace prefix, an abbreviation for a namespace URI.

XML Namespaces

The purpose of XML namespaces is explained clearly in Section 1, Motivation and Summary of the W3C Recommendation: Namespaces in XML 1.0 (Third Edition):

We envision applications of Extensible Markup Language (XML) where a single XML document may contain elements and attributes (here referred to as a "markup vocabulary") that are defined for and used by multiple software modules. One motivation for this is modularity: if such a markup vocabulary exists which is well-understood and for which there is useful software available, it is better to re-use this markup rather than re-invent it.

Such documents, containing multiple markup vocabularies, pose problems of recognition and collision. Software modules need to be able to recognize the elements and attributes which they are designed to process, even in the face of "collisions" occurring when markup intended for some other software package uses the same element name or attribute name.

These considerations require that document constructs should have names constructed so as to avoid clashes between names from different markup vocabularies. This specification describes a mechanism, XML namespaces, which accomplishes this by assigning expanded names to elements and attributes.

So, in RDL as with in other standards efforts, namespaces allow the creation of a vocabulary without worry of naming collisions with other vocabularies.