As you can see from the above screenshot, the maximum password age is set to 90 days. This means that if I have a login that uses SQL Server Authentication or a contained database user with a password, if the login’s or user’s password was last changed 90 days (or more) ago, then the login/user will be prompted by SQL Server to change her password.
Even though this practice was used for many years, not only in SQL Server but similarly, on Windows-level, as well as in other systems and applications, recent studies argue that it should not be a recommended practice anymore. On the contrary, these studies suggest that user passwords should not be regularly changed but rather change only when there is a specific and justified reason to do so. Furthermore, new security standards are being formulated that contain new recommendations on password change.
Security is all about trade-offs. If you make people change their passwords every 90 days, they’ll write the password down somewhere.