Srb or Serb means of the same kin, or of the same people. In a wider sense an old umbrella term for those people sharing the same origin or speaking the same language. This is the traditional way the term is used and understood. The term is today also sometimes used as a synonym for 'brother', denominating those Slavs that aren't necessarily of our modern ethnicity but share the same ideals thus standing on the same side, as in 'moj Srbin'.
There are theories regarding the origin of the name of the Serbs, the most likely one is that it is derived from the Old Slavic root *serb-, meaning "same". The root can still be found in words of other Slavic languages; pol. Pasierb/Pasierbica, ukr. Пасерб – Step son/daughter, a child that was made to belong to the same kin.
Sebar was the peasant class in the medieval Serbian class system. Those people were also sometimes called Serblji or Serbs in the documents, meaning common folk.