You have some unique item you must have to work your magic. This can be a staff, diadem, dowsing pin, special hat, gizmo or whatever, but without it, you are powerless. Unlike HocusPocus, this is one specific item, not a set of props.
This focus can be either destructible or indestructible. If it is indestructible, it cannot be replaced; you MUST get your unique item back. If it is destructible, it is still tough and hard to destroy, but it can be damaged, and you can replace it.
Your focus is a signature item; it will not disappear unless there is some plot-driven reason for it to disappear, and customs officials and other bystanders will generally ignore it. But its function is obvious to other sorcerers and to everyone once you start using it in obvious ways. Those in the know can try to take it away from you.
Most foci are large and reasonably obvious, like a staff. If your focus is small and unobstrusive, like a ring, you are prevented from recovering Magic points when you don't have it.