Well, I never heard of it, but Uncle Google says it's a bisphosphonate, which I do know about.
The bones in your body are continuously renewed, by your body dissolving out the calcium, and replacing it. When you are on hormone therapy (and for some women after menopause), the loss of testosterone (estrogen in women) significantly slows the replacing of bone, so your body continues dissolving it away, but doesn't replace it as fast, so bones become less dense and weaker.
There are two ways to treat this - try and make sure your body has all the calcium and vitamin D it needs, but in the absence of hormones, than only works when you are stressing the bones with weight bearing or shock/impact exercise.
The other way is to reduce the body's rate of dissolving out the calcium in the first place, and that's what bisphosphonates do.
The other thing is might be used for is if bone mets are causing release of too much calcium into the blood, as it will reduce the calcium coming from your bones.