Hard Mouthed Horses are horses that resist the bit, have very stiff jaws, and wont accept the bit, completely ignoring your cues.
There can be several reasons to why a horse is hard mouthed, the most common reason would be more of a phsicological problem than a physical one. This is mostly cause by bad riding or failing to train the horse to give in to pressure. Also this could happen to well train horses, when the rider depends too much on the reins for balance, and does not use his/her legs and seat to steer the horse; Pulling a horse too much too often can only develop a hard mouth, stiffness and resistance. Horses by nature resist to pressure, so its our goal to teach them not to.
A lot of riders will only put a harder bit masking the problem, but in the end the horse will eventually learn to resist the bit, forcing the rider to use even a harsher bit, its like a cycle that will continue to repeat itself until it gets fixed from the roots. Hard-mouthed horses also stiffen up when you try to bend or flex them, causing flat turns and jolting transitions. Often, the very things that horses do to lessen the pain cause riders to stiffen up more and pull harder on the reins.
A hard mouthed horse can be fixed, by careful retraining, going back to basics, and teaching the meaning of "whoa" rather than just pulling or seesawing the bit un till he cant resist no more, Teach your horse the meaning of the seat, horses are very sensitive animals, they can feel the slightest touch of a fly on their body; Your goal as a rider is to each your horse the real meaning of your body cues and avoid pulling your horse too hard, it just creates resistance.
There can be other things causing a hard mouthed horse, such as back and mouth problems. Horses who have back problems will try to pull on the bit in an attempt to lift their back and the rider trying to get rid of the pain.
"There is something about the outside of a horse that is good for the inside of a man." -Winston Churchill