Meaning... A spy can kill a guy who has about the same or less ability to protect himself (personal security). So if a spy has 5 skill and a guy has 2 personal security, the spy will kill have a good chance of killing him. If I guy has 15 personal security, an assassin would need like 20 skill to be able to kill him easily. However, agents can only get a max of 10 'agent skill'. So, once a character gets a few points of personal security, he is basically invunerable because it is impossible for an agent to match him in skill.
(This is just an example - I don't know if these stats compare 1:1.)