Firsthand knowledge – in the court procedures this phrase refers to the personal understanding of circumstances and events, which is re-told and explained by a witness in a courtroom, during a court hearing on the concrete lawsuit. In other words, the witness has been an eye-witness of these circumstances or events and so has accepted it directly. Therefore they have clear and detailed personal knowledge of it. An example is a statement in affidavits.
A synonym is “personal knowledge”.