The Bears tried running a fake punt with 10 men on the field and in an illegal formation
It did not work.
The Saints are not good. However, they are playing significantly better than the Bears, who have eclipsed the Saints not-goodness by being legitimately bad. This play encapsulates that:
The Bears had only 10 guys on the field, leaving one less to block, which contributed to them not picking up the first down. Having 10 guys on the field isn’t in and of itself illegal, but they also didn’t have the right guys in the right places, which is an illegal formation penalty.
The Saints declined, obviously. And with their great field position, they… punted the ball into the end zone. Great game, this Monday Night Football showdown.