Relative pitch is the ability to determine a note based on a recently heard reference. For example, being able to sing an E after having a concert A played on a piano. This is largely a learned skill, some people are better than others at it, but almost everyone can improve with practice. The key is to learning the intervals between notes so that you can establish a relationship between a note you hear and the note you wish to produce. This is opposed to perfect pitch (absolute pitch), which is the ability to be able to produce any note without any reference, which in my experience relatively few people are capable of doing.