Diplomatic Immunity: One of the most popular instances of this people will remember from Lethal Weapon II, but what exactly is it?

dip·lo·mat·ic im·mu·ni·ty
ˈˌdipləˈmadik iˈmyo͞onədē/Submit
noun
The privilege of exemption from certain laws and taxes granted to diplomats by the country in which they are working.

Today's rules surrounding diplomatic immunity were originally laid out in 1961 at the Vienna Convention on Diplomatic Relations, with 191 different nations ratifying the treaty. But let's ask a bigger question: is it really a get out of jail free card?

Source
Kind of? If a person with diplomatic immunity attempts to endanger public safety, law enforcement is allowed to intervene to prevent a crime from occurring. The diplomat can't be arrested for having attempted the act. Diplomatic Immunity is only granted to very high ranking officials, though. Most others will have "functional immunity" which will only apply to their work, if someone with functional immunity were to enact a crime outside of their working function, they can still be arrested. Now if the host nation of someone with diplomatic immunity decides to remove it from them for some kind of action, they can be prosecuted by the host country when returned or even in the country they are currently in. These are extreme cases. There's a bit more to the story, but the point is that Diplomatic Immunity might not always be able to save a diplomat should they do something despicable and unjust.