A more thorough explanation:

Quamdiu bene se gesserint is a Latin term used in law. It means "as long as they shall conduct themselves properly." This term is often used in reference to holding an office, such as a judge's tenure.

For example, the Act of Settlement in 1700 provided that a judge's tenure was no longer at the king's pleasure, but could continue until death or improper conduct. This means that judges were protected against arbitrary dismissal as long as they behaved properly.

The singular form of this term is quamdiu se bene gesserit, which means "as long as he behaves himself properly."

Overall, quamdiu bene se gesserint is a way to ensure that those in positions of power are held accountable for their actions and behavior while in office.