This expression is used by many to indicate that an exception in some way confirms a rule. Others say that the exception tests the rule. In its original legal sense, it meant that a rule could sometimes be inferred from an exemption or exception. In general use, the first meaning predominates nowadays, much to the annoyance of some pedants.
If someone is said to be the judge, jury, and executioner, it means they are in charge of every decision made, and they have the power to be rid of whomever they choose.
If something's a moot point, there's some disagreement about it: a debatable point.
In the U.S., this expression usually means that there is no point in debating something, because it just doesn't matter.
An example: If you are arguing over whether to go the beach or to the park, but you find out the car won't start and you can't go anywhere, then the destination is said to be a moot point.
The word of the law means that the law is interpreted in an absolutely literal way which goes against the ideas that the lawmakers had wished to implement.