The master password is the password for the secure keystore . When a new application server domain is created, a new self-signed certificate is generated and stored in the relevant keystore, which is locked using the master password. If the master password is not the default, the start-domain command prompts you for the master password. Once the correct master password is entered, the domain starts.
When a node agent associated with the domain is created, the node agent synchronizes the data with domain. While doing so, the keystore is also synchronized. Any server instance controlled by this node agent needs to open the keystore. Since the store is essentially identical to the store that was created by the domain creation process, it can only be opened by an identical master password. But the master password itself is never synchronized, meaning it is not transmitted to the node agent during the synchronization, but needs to be available with the node agent locally. This is why creation and/or starting of a node agent prompts you for the master password and you need to enter the same password that you entered while creating/starting the domain. If the master password is changed for a domain, you will have to perform the same step to change it at every node agent that is associated with this domain.