These are settings for the config file. They determine the system configuration of how phpList works on your system, and once set, cannot be altered in the administrative pages.
They are organised by variables and constants. The choice between variable or constant is historical, but the effects are different. A constant, once defined cannot be changed again, even if it is defined again later on. A variable, once set, will be overwritten in value when it is set later on. This may be something to be aware of.
Constants look like
define('SOME_CONSTANT','The Value of the constant');
Variables look like
$aVariable = 'The value of the variable';