Lameness means the horse is sore somewhere on it's body and can't walk, trot, or run properly.
Lameness can vary from barely noticeable to the horse is literally only walking on three legs.
You could be "lame" from a blister wearing on your foot because your new shoes don't fit right, to getting hit in the knee cap while standing on base and you missed the baseball someone threw to you. Or hurting your back because you fell on ice. Any injury or disease that stops you from walking properly is a cause for lameness
While Wikipedia isn't always the best place to get information, this link is fairly concise and should give you some idea of things that cause lameness in horses. http://en.wikipedia.org/wiki/Lameness_(equine
Hope this helps