The law of the tribe states that if someone doesn't understand what they are seeing, they will either attack it or ignore it. Basically what this means is that if someone does not want to put the time in to understand your mindframe or ideology for example, they will not take the time to educate themselves because it takes to long. Instead, they may just attack it because it's the path of least resistence for them to draw a conclusion and move on with whatever it is they were doing. It is also their way of attempting to extract power from a powerful individual.
For example, people don't want to believe that the united states government may have had some involvement in the Kennedy assasination or the September 11th attacks, so they attack people that do and call them "Conspiracy theorists".
This is also known as ad hominen or attacking the attacker instead of attacking the issue. Because they are a member of the tribe that doesn't understand what they are seeing, they attack it, because they don't know if what they are seeing could potentially pose a threat to them or not. On the flipside, people may have heard of these alternative viewpoints on those events but choose to ignore them because they don't know enough about it aka, they don't understand. Law of the tribe lets them do that.