A dialer code is a sequence of characters that performs a pre-defined action when entered in the phone's dialer (the keypad). The sequence consists of digits delimited and enclosed by asterisk (*) and hash (#) characters. These codes allow users to access information, control the connectivity of their device, and access troubleshooting/technical menus that are typically hidden or inaccessible without them. Android devices (and many other OSes as well) have some sort of dialer codes to force actions and view hidden menus.
For users with Google Fi, the dialer codes interact with the Google Fi app in order to allow you to switch between carriers. Though, please note, Google Fi will automatically revert back to choosing to which carrier you are connected if two hours have passed since you manually changed carriers.