The circle with a 100 cm radius around the net. A serve landing on or inside it is a fault — the zone is what keeps serves returnable.
Without this zone a server could push the ball onto the net from point-blank range so flat that returning it is simply impossible. So the 100 cm circle is not an arbitrary restriction, it is what keeps the serve returnable at all — much like the rule that the ball may not reach the receiver above their shoulder height in an athletic stance. The typical mistake happens on the second serve: instead of playing it safe, people hit it too hard — the double fault is one of the most common beginner errors in the game, according to the community. Play your second one long and placed rather than hard. What often gets missed: the zone is not only a serve rule — the final hit of a rally also has to be initiated from outside the 100 cm.