The master daemon is a supervisory application which controls and monitors all of the other Postfix processes. The master.cf file is the master daemon's configuration file. The master.cf file is the throttle for Postfix. Here you set all of the daemon process count limits. A good example of a useful limit would be to set a limit on the number of SMTP processes which can be executed simultaneously, after all, you might not want to receive 50 messages inbound all at the same time. The key thing to understand here is that any process without an express limit defaults to a 50 process limit.
In general terms the master.cf file is fine with the defaults as they are so you can leave it alone.