Relevant sources are those that contain information directly related to a particular topic. Too many students write poor essays because they "force" their sources. That is, instead of reading numerous sources and using only the relevant ones, they rely on sources that mention their topics but do not contain detailed material that supports their theses. An essay built on this kind of cursory research is not a coherent argument, but a hodge-podge of miscellaneous findings and observations. When you come across a resource, make sure that it contains material that will help you to explain your topic or support your thesis. If not, discard it.