|This page in a nutshell: While all users are asked to maintain a strong password, some users with advanced permissions are required to do so and the strength of their passwords may be audited by the Wikimedia Foundation|
Although Wikipedia:User account security has contained standard advice for password strength for some time, the English-language Wikipedia did not have password requirements for its first 14 years. In late 2015, there was a security breaching incident involving users with advanced permissions that led to a security review. That review resulted in password requirements for some users with advanced permissions, and advised changes to global policy and auditing and enforcement by the Wikimedia Foundation. In addition to the local policy, the Wikimedia Foundation has now created a global policy at meta:Password policy.
Enforcement and auditing
Users with advanced permissions who are found to be out of compliance with these requirements may have their permissions revoked until they have made adequate assurances that they have rectified the issue. Users who repeatedly fail to maintain a strong password may have their permissions permanently revoked by the Arbitration Committee.
So that's it, my account is secure?
No, not really. A strong password and password security are just one part of securing your account. Users with advanced permissions, and indeed all users, should be taking steps above and beyond these requirements to insure the security of their accounts. Two-factor authentication is now available to administrators and will hopefully be rolled out to all users in the near future. Simply logging out when you are done for the day if you are using a device that there is even a possibility another person will have access to is another basic security measure. Avoid "recycling"; your Wikipedia password should be unique and not used to log in anywhere else. Failure to abide by this simple precaution has led to numerous security breaches over the last several years. A committed identity can help you prove you are the legitimate account holder and assist you in regaining control of your account if it is breached. More information is available at WP:SECURITY.