Yep, chain repair tool. But I will disagree with Moab's comment.
Though it is hard to judge from the angle of the pictures, this chain tool looks a little weird. On most chain tools one of the two ridges (usually the upper one closest to the handle or screw) is thicker than the other. This is because the chain is set on the upper ridge to loosen a stiff link once the pin has been pushed into place with the chain link sitting on the lower ridge. The below utube video is a good lesson on using a chain tool to repair a chain:
One thing not emphasized on the video that should be. When breaking the chain in two, leave the pin sticking up just half a millimeter or so under the roller (you will have to GENTLY twist chain to pull it apart) - this makes it easy to insure that the chain will be properly aligned when the new pin is pushed down. Also, it is best only to only break a chain at a particular pin only once - if you do it more than once, that pin will enlarge the hole and never fit snugly together again. Shimano recommends that pins on their chains should never be re-used. however, YMMV.
The practical way to tell if it is the right size tool for your chain is fairly simple. Set a chain link with the lower ridge in between two rollers (the cylindrical part of the chain). The chain should fot easily, but straight with a minimal of wiggle, so that the chain does not twist or bend when the screw is turned and the pin is pushed downward. If you have any difficulty at all keeping the chain straight and at a perfect right (perpendicular) angle to the pin when the pin is being pushed down, it is the wrong size tool, the tool is designed for a newer, narrower chain than yours. If the chain does not easily fit onto the ridge, don't force it. In this case most likely you are probably trying to put a newer narrow chain (9 or 10 sprockets on the rear wheel of the bike) on a tool made back in the day when chains were 8, 7, 6 or (if you are a really old) 5 sprockets on the rear wheel were standard.