In 2013, hackers accessed Yahoo user accounts and were able to easily crack encrypted passwords because many people reuse passwords across multiple accounts. The document outlines guidelines for strong passwords that are at least 8 characters long and include a mixture of uppercase and lowercase letters, numbers, and special characters. It also recommends using a password manager to store unique passwords for each account, but ensuring the password used to access the manager is very strong. Regularly changing passwords and using multi-factor authentication provides additional security.