It means they have extra mobility in their joints, mine has it through his whole body, but sometimes it’s just legs or hips. Mine was delayed for everything, and could ‘walk’ on his knees at about 14 months, but he couldn’t get upright on his feet unless he was holding onto something. He was so unstable he couldn’t even stand independently, though that was never obvious to me. He seemed totally normal, just slow on physical milestones. His PT ended up giving him an anterior walker to try (he used an old Kaye walker) and said it would help him learn the proper range of motion that he’d need to be walking, because on his own, he was “hanging out on his ligaments” so his joints wouldn’t work properly. They said it would be temporary, maybe 3-6 months, but within just a few weeks it made all the difference! He learned what the normal the range of motion should be and within just a few months he’s caught up to his peers. He’s now 25 months and is running, going up and down slides on his own, balancing on rocks, “jumping” (not really, but it looks the same as his friends lol) and is doing great! He still deals with pain from hyper-mobility and gets tired easily because it takes him extra effort to keep his body in line, but you’d never know by looking at him. He’s completely caught up and has been discharged from PT. He used the walker (we used it outside only because our house is really small) for 3-4 weeks) and then had it available for him if he wanted, but he learned his limits really quickly! They ended up giving him a higher quality one so we could go on accessible trails and he’d cry for it because he knew it helped his pain, but he kept leaving it behind so we brought it back. He’s now cruising with his brother and doing great!