While going through features of .NET framework 4.5, I found that it supports RFC-compliant URIs. What does it mean to have RFC-compliant URI support?
RFC is a set of internet standards that define various aspects of internet protocols - in your case the standard format of an URI. You can find the relevant standard here: http://www.ietf.org/rfc/rfc2396.txt
RFC 2396 defines explicitly what are the parts that make up an URI (for example the scheme, authority, path etc.) and what are the valid values for each of them.
What they mean with RFC-compliant URIs is that the Uri class from the .NET framework follows the above standard and enforces it.