I think this quote has been used too many times in movie about lawyers or better known as legal thriller. The importance of your capability to prove that something has happened. In contrast with just knowing that something has happened. Especially if it is based on something or someone which you have not been able to bring to light. A missing evidence. Or a missing witness. Two important tools for a lawyer in a courthouse.
The whole quote goes :
"It is not what you know, it is what you can prove in court".
It was said recently in the movie Law Abiding Citizen as the prosecutor played by Jamie Foxx was outwitted by the revengeful Gerard Butler. It was a great movie which used the courtroom as the background but still couldn't beat the Edward Norton's multiple personality character who beats Richard Gere's character in Primal Fear or even A Time To Kill, either the book by Grisham or the movie with McConaughey and Bullock. Watch it all if you have not.
All these movies are more of a legal thriller than a courtroom drama. But good nonetheless.