There are several methods: squish it, hack it, dust it, drown it, or coddle it for a very long time in gentle, moist heat.
There are various circumstances that affect how tender a piece of meat is. The length of muscle fiber is what makes meat tough, so the cut of meat you choose is particularly important. How the meat is handled after the animal is killed also has a great impact on whether it is tough or tender. If the meat has been handled properly, it will be juicier, resist spoiling longer, and have a better texture. But of course, you have no control over that.
So, once you get your tough cut of meat home, you’ve got to find ways to break up or soften the muscle fibers. Using mechanical means, you can flatten it with a tenderizing mallet, the bottom of a heavy pan or some other blunt object (most people place the meat between two pieces of waxed paper or plastic wrap to keep it tidy), you can score the meat across the fibers with a sharp knife or use a tenderizing tool, or, in dire cases, you can grind it and turn it into hamburger. Once the meat is cooked, you may have another opportunity to physically tenderize it. A flank steak, for instance, is cut in the direction of the muscle grain and is inherently tough. Once cooked, you always slice it thinly against the grain, which transforms a tough piece of meat into a still somewhat chewy, but very edible, serving.
There are also chemical solutions - some better than others. There are a variety of powdered tenderizers on the market (some instant, some requiring a period of rest). These include a powdered form of papain, an enzyme found in papaya, which has been used for this purpose for centuries. An awful lot of cooks have various reasons for avoiding commercial powdered meat tenderizers, though. They can make the outside of the meat mushy while leaving the inside tough, leech out the juices, and impart an unpleasant flavor.
A widely accepted alternative (and the basis of many cookbooks) is to marinate the meat, which not only tenderizes somewhat, but adds flavor. To be an effective tenderizer, a marinade must include an acid, which "denatures" or unwinds the long proteins in the muscle. Marinades also may not get inside the meat very well, leaving it tough in the center. Some recipes call for you to inject the marinade into the meat at various points to get below the surface, and there are injectors on the market for this purpose. Dry marinades and rubs impart flavor, but do nothing to tenderize the meat.
Finally, for large tough cuts of meat, the best solution is long, moist cooking - stewing or braising. That is the reason pot roasts and stewed hens spend hours in the oven.