Question about little pro and variables

Hi, I have some questions that googling has been unable to help me with. I've been reading articles about pro-drop in children learning English. The idea is that children speaking English initially have a misset parameter, to topic-drop (like Chinese). To account for the fact that object drop in English speaking children is rare, Hyams (1991) proposed that "null elements includes pro but not variables." I am unclear in that context, what variables means. Any help would be appreciated. Linguistics is not my primary field of study, so I realize this may be a very basic question.