Windshield wiper arm on a 2002 Chevrolet Impala flops around. Is it stripped or is there a nut i could tighten?

Your nut has come loose - but your wiper transmission is going to look stripped. Simple fix --- remove the nut and the wiper arm. Take a sharp pick (I prefer one with a 90 deg. angle on it) and pull the pick vertically on the wiper transmission stud (just below where the nut came off). You will notice a whole bunch of fine grooves in the transmission stud. Clean all of the grooves out. Reinstall the wiper arm in its original position. Next apply some high-strength threadlocker (usually red in color) to the nut. Tighten nut securely. This situation is normally caused from either loose nuts on the wiper transmission or from excessive force being applied to the wiper arms, such as forcing them across a windshield that is covered in snow/ice. I suggest repeating this procedure to the other side as well just as a precaution.