Someone able to recount an event they witnessed or took part in has "first hand experience". First hand experience of the "you had to be there" variety can seem especially valuable and privileged, but it often remains potentially subject to errors in sense-perception and in personal interpretation.
Second-hand experience can offer richer resources: recorded and/or summarised from first-hand observers or experiencers or from instruments, and potentially expressing multiple points of view.
Third-hand experience, based on indirect and possibly unreliable rumour or hearsay, can (even given reliable accounts) potentially stray perilously close to blind honouring of authority.