In MATLAB, PID controller tuning for a buck converter involves optimizing the proportional, integral, and derivative parameters to regulate the output voltage of the converter efficiently. The PID controller adjusts the duty cycle of the buck converter based on the error between the desired output voltage and the actual output voltage. Tuning the PID controller typically involves using techniques such as Ziegler-Nichols, Cohen-Coon, or trial-and-error methods to find the optimal values for the proportional, integral, and derivative gains. MATLAB provides tools such as Simulink and Control System Toolbox for simulating the buck converter system and fine-tuning the PID controller parameters to achieve desired performance specifications such as stability, overshoot, and settling time. By employing MATLAB for PID controller tuning, engineers can streamline the process of designing robust control systems for buck converters in power electronics applications.