The question mark denotes the start of parameters you are passing to the script. Subject is the first parameter. The '=' denotes an assignment, meaning subjects value is going to be whatever is in mailSubject. The ampersand denotes the start of a new parameter, in this case "body." Body's value will be whatever is in mailBody. You denote any number of new parameters to pass with more ampersands, for example:
someurl.com/somescript.php?parameter1=somevalue¶meter2=othervalue¶meter3=anothervalue