If you walk into your player's room and have to ask that question, then first check under the bed for last week's meatloaf. If that's already been devoured by the dust bunnies then you may want to get a gas mask and venture towards the hockey bag. That stench is mildew, bacteria, enzymes and dead skin stewing in natural and synthetic fibers. You'd likely gag at the list of nasty critters that are colonizing the hockey bag.
IS IT HARMFUL?
A small percentage of players will experience rather nasty skin irritations from contact with their gear. It could be as simple as an allergic reaction all the way to a full blown infection. There are cases of infection because of hockey gear, even relatively clean gear. That's an important reason to wear undergarments under pads. So, Yes, not only is it really foul, it can be harmful.
HOW DO I TURN IT OFF?
It's not practical to wash the gear after every use, in fact, doing so will shorten the pads' effective life span, by breaking them down. The most effective control is to allow the pads to dry out completely after use. Do not store gear in the bag. If you store them even a little damp for even a little while, then you aren't controlling anything. There are many commercially available gear deodorizers. Most of them work to different degrees. All of them suggest that the gear be allowed to dry completely after application of the product. Helmets should be rinsed before drying. A shower head for a few seconds will do very well. Cups and supporters can be washed in with the whites with bleach. Washing gear in a Front Loading washer will be better than a top loader... we are talking about hockey gear though and it is made to take a beating so don't think that a trip to the coin-op laundry is the only way to wash the gear. Shoulder Pads, Elbow Pads and Shinguards can also be washed in the dishwasher without detergent (too much bleach), on the top rack and without the dry cylce (too much heat). The dishwasher doesn't work as good as the washing machine for a thorough cleaning, but there's also no agitation to break down the equipment.
ANY OTHER REMEDIES?
The gear bag deodorizers will help to some degree, the commercial deodorizers will also work to some degree, but all that will end up costing quite a bit of money over the course of a season. You could wash the gear in a gentle cycle (wouldn't want the pads to get roughed up or anything) with detergent and a capful of Pine Sol about once a month. In between washes you could spray the gear down with products like Febreeze. Before drenching your players' gear in some flowery stuff, please check to make sure that your player won't experience a bad reaction to the flowery stuff. Lysol is known to cause reactions with many players, and it doesn't improve the "hockey bag stench", it just changes it into something worse.
Here's a home solution that works for many. The recipe has evolved over many years with new products coming out but the principle ingredients remain the same. A very dilute bleach water solution in a spray bottle. That's it. You could use Spring Fresh Scented Bleach and you could use Evian Bottled water if you like, but they aren't necessary. If you can smell the chlorine on the gear then you didn't dilute it enough. Don't apply it to wet gear, apply it to dry gear just before you put it away. Don't drench the pads, simply spray a light mist on the pads and allow them to dry. Don't forget to spray the inside of the bag. If your player uses a polycarbonate visor on her helmet, then you shouldn't get the bleach solution on the visor.
Drying gear in the sun is probably the best method to kill the "cooties" and get rid of some of the smell. Even if the sun isn't practical.... fully drying the gear will help out a great deal. It will dry out completely between practices and games so long as you hang it or lay it out soon after practice.
One last method is freezing the gear to kill the bacteria. Place the gear in trash bags and freeze for 12 hours and then let them thaw and dry for 12 hours. If that didn't work then combine one or several of the other methods. No need to get carried away.
Baking soda helps eliminate the odors but makes a mess and doesn't kill what causes the smell. Charcoal/Carbon gear bag deodorizers also only absorb the smell, but do not kill the bacteria.
Methods such as stopping at the car wash on the way home from Practice (with your player still dressed in gear) has proven to be entertaining but not effective for killing the odor.