A Clean Desk Policy is a physical security measure requiring employees to avoid leaving any paper on their desk in plain sight, and instead place it in a locked cabinet when they leave their desk.
The motivation behind a clean desk policy is to make it more difficult for passersby (for example: after hours cleaning staff) to steal or photograph sensitive information such as employee records, customer records, login credentials, or intellectual property.
A clean desk policy is also a key component of any security training program since it is an easy way to build awareness of sensitive information handling among employees from all departments.
The SANS institute has a sample Clean Desk Policy available for free use.