Credential stuffing takes advantage of people reusing username and password combinations across different accounts. By fraudulently gaining valid combinations for one site, and successfully using them on other sites an attacker can access legitimate accounts. The primary motivation is financial, but it can lead to identity theft.
For example users who use the same username and password across multiple websites are more likely to suffer from credential stuffing attacks.
It is recommended that you use a unique secure password for each website that you use.