In Francesco Redi's experiment, the variable being tested was the presence or absence of air exposure. Specifically, he compared uncooked meat that was left uncovered (in contact with air) to meat that was sealed in jars (with no air exposure). Therefore, the key variable was uncovered meat that had contact with air.
Redi's experiment was designed to demonstrate that maggots formed from eggs laid by flies, rather than spontaneously coming from the decaying meat, which provided evidence against the theory of spontaneous generation.