Threshold works on an introduction system. This means that barring celebrities (see hegemons below), you will by default not know anybody. It is through the gradual interaction with people, introducing yourself to them and they in turn introducing to you, that you begin to build your network of people you know in society.
A stranger is someone who you do not know (obviously). When you walk into a room and you see a player's short description as "a male elf" this means you do not know their name. You will not know their name until they introduce themselves to you. Even though emotes, enter/exit messages and other commands will reveal their name to you, you cannot roleplay that you know their name or call them by name because your character does not know it.
Your first level of introduction to someone is a casual introduction. This is merely a passing of your name to another so that they will recognize you. After a person has casually introduced to you, you will see their name and their title show up when you look in a room or at them.
The second level of introduction is a full introduction. You should only do a full introduction to someone whom your character would trust as such an introduction allows them access to your brain and other abilities in the game will rely on this recognition to function. A full introduction allows access to the tell command if both parties have fully introduced to each other.
Hegemons are powerful mortals who are famous and as such gain the ability of instantly recognizing and being recognized by everyone.