The Silence

(See The Pact for the context of the Silence)
Fundamentally, the Silence is the enforcement mechanism for the Pact. It is magically a nuanced geas, binding the person who makes the oath to certain things as stated in the explict oath taken. There is a standard oath taken by everyone with magic (who wishes to remain in the magical community) at the age of 12, that protects the magical community.
If you come close to breaking an oath on the Silence, you will feel the effects - whether you meant to break it or not. (In other words, it is not driven by your intention.) This could be by doing obvious magic near someone non-magical, or it coudl be by breaking an explicit oath on the Silence.
Some professions use them routinely (the Guard and Penelopes, some Healers and nurses), but other people may go their entire life making an oath on the Silence no more than two or three times.
(Think of it about like you might making a solemn oath to tell the truth in court: some people do this routinely because of the nature of their work or their life choices, but many people might only do it if called for jury duty, or very rarely giving evidence in some specific circumstance.)

What are the effects?

Fundamentally, you will feel your greatest fear pressing on you.
It is meant to be deeply distracting, to turn you away from whatever you were doing that might break your oath.
It is not directly meant to be harmful or damaging in a long-term sense, but someone who presses through that fear (or who sets themselves up to break the oath and then can't undo it directly), will feel the consequences. (The Hare and the Oak gets into this.) Pressing past the fear can be deadly, due to the strain it puts on the body, or it can be debilitating (forcing you to pass out or be otherwise immobilised prevents you from breaking the oath.)
The fear you feel can change during your life.
For many of my characters, their greatest fear is now related to their experiences in the War, for example, and before that it was something else. Making an oath on the Silence brings a brush of whatever the fear is, a momentary flicker of knowing what you'll feel if you get too close to the bounds of the oath.