You will see references to "constants.php" throughout our guides. The constants.php is a special file you an optionally create in the root directory of your Suite which can include special commands and configuration options which alter the normal behavior of the Suite.
You would create a file in the root directory of your Suite, the same directory where index.php and init.php are located, called constants.php. In that file you would simply enter:
Then below that line any special commands referenced throughout our guides or via instructions in the AdminCP. Many functions are available and common settings include caching controls, debug tools, and special overrides based on your server environment.
Most people do not ever need to worry about constants.php until you find a special circumstance.