During a Quiet Period, a publicly listed company cannot make any announcements about anything that could cause a normal investor to change their position on the company's stock. Normally, that means the company does not discuss any of the following:

New deals or wins signed in that current quarter.
Announcements about previously sold implementations going live are allowed but must be explicitly described as such.
Management changes
Progress against company goals
Major product or service announcements
Major partnership announcements