A PLC HMI is a user interface that allows operators to interact with a PLC, which is a computer-based control system used to automate industrial processes. The HMI provides a graphical representation of the process, allowing users to monitor and control the system. To prevent unauthorized access and ensure the integrity of the process, PLCs and HMIs use passwords to restrict access to authorized personnel. These passwords protect the system from malicious activities, such as tampering with process parameters, modifying code, or disrupting operations.