Well, I'm not exactly an anatomy/physiology expert, but my understanding is that, unless severe, it is generally not something to be worried about in regards to soundness. I've never heard a term besides "over at the knee," and I've never read a truly technical article about it, but all the information I have read agrees that it is a far less worrisome fault than "back at the knee" / "calf-kneed," which is the opposite problem. I think a horse that is badly over at the knee is somewhat more prone to soft tissue injury, but milder cases are more cosmetic. In other horses, it can be a sign of hard work in the past, though I don't know what the mechanism is that causes the knees to "buckle" over time. You see it a lot in older TB's.
Above: The horse at the left exhibits calf knees or backwards knees, a severe conformation fault with potential lameness implications if used for strenous athletic work. The middle horse shows perfectly straight legs with solid strong bone structure. The horse at the right is slightly bent over at the knee, perhaps caused by hard use earlier in life. This is more consmetic in nature than truly detrimental. All horses pictured above have good angle to their pasterns and adequate length, though some disciplines might like them to be marginally longer for smoothness in gait. The middle horse has good ratio of bone length from upper to lower leg (short cannons are ideal for greater speed and fluidity of movement). The horse at the right has slightly longer-than-ideal cannon bones.
That's the image I use a lot because the horse on the left has about the worst example of calf knees I've seen. The horse on the right is my teenage gelding, who was used pretty hard in the past. He's only slightly over at the knee, but I suspect that his may have been caused more from hard riding than genetics.