If your deck railing is loose or wobbly, it is probably the result of a weak rail post connection to the deck frame. Posts should be connected to the deck framing with lag screws or bolts at a minimum. Screws or nails are not generally acceptable. If your rail posts are mounted to the interior of the frame, you can add strength to the rail by installing blocking around the post to trap the post in a pocket. If your railing posts are connected to the outside of the frame, you can try to tighten the bolts and install blocking to the rim joist to stiffen the rail.