Relativity of norms is a concept regarding normative scores and how they differ when compared between different groups. For example, a student takes a standardized test which is scored on a normative scale (like the SAT). They score in the 80th percentile nationally (which means they did better than 80% of all students who took the test nationwide), 85th percentile locally (which means they did better than 85% of the students who took the test in their school system) and 55th percentile of applicants to a prestigious university (they did better than 55% of the students that applied to that school).
These are all normative scores but they differ between the groups. Relativity of norms is a concept of determining which group is most relevant to what information you want. If the information most important is how the student compared to others in their area then the local scores would be used. If the student wants to attend the prestigious university then those scores would be more important. This concept just addresses how normative scores can be different and how the same score can be viewed differently depending on which sample is looked at.