MPIN stands for "Master PIN Code" and is a 4-digit secret code that is required for each transaction made from a JazzCash account. It is a security password that protects the account from unauthorized access and is set by the user when creating a new account through the JazzCash app. The MPIN is separate from the ATM PIN and should not be used to attempt a withdrawal from an ATM. If a user forgets their MPIN, they can reset it by dialing 786612# or by calling 4444. It is important to keep the MPIN secure and not share it with anyone to prevent unauthorized access to the account.