OK, so "chain of custody" is a concept that is very critical when handling high interest items where ensuring that unauthorized people don't have access to them is critical. This concept is used both in evidence for crimes, but also throughout the handling of classified materials.As for the 5th I think it's a bit of a mouthful and to be honest I don't quite understand it.
To establish a chain of custody each person who's had access to and/or control over the items has to be recorded and noted from time acquired.
The reason this is critical with criminal evidence is to ensure that the evidence is not tampered with and also show that the evidence is directly related to the crime in question. You can't just present ANY bloody knife in a court case for a knife assault, you have to present the knife that was actually at the crime scene and a proper chain of custody record shows this. If the police and prosecutors fail to do this, a defendant can get the evidence thrown out.