Okay, trying again... I wrote out a great response last night, but it got lost in cyberspace and I was too tired to do anything about it.
Your doc gave you three separate diagnoses there, two of which may be linked. The good news is that you probably don't have to worry too much. I'll try to explain things as much as I can, and you can probably search the web for relevant pictures as well.
1. Hypoplastic ribs. Hypoplastic just means "not enough growth." So your ribs just didn't grow as much as would be expected. If you are just getting this diagnosis now, it probably isn't causing any problems. Of course, I don't know what symptoms prompted you to go to the doc, so it's possible. But it's probably just one of those things that you've had all your life and was just discovered now when you went in for something else.
2. Inflammation of the sacroiliac (SI) joint. The SI joint is the joint between the sacrum and the ilium. The sacrum is the last 5 vertebrae in your spinal column that are fused into a single bone. If you go about hip-level and put your hand on your backside in the middle, you should be just about over where your sacrum is. And it's actually about the size of your hand. The ilium is a particular portion of your hip/pelvic bone. You have two of these bones, one on each side. Each hip bone has three portions which started out as separate bones but fuse together into a single bone by puberty. The bones are the ilium (in the top back), the is****m (in the bottom back), and the pubis (swinging around to the front). The two hip bones and the sacrum make up your pelvis. You have the sacrum in the middle in the back, then the hip bones join up on either side. The hip bones sort of curve around and meet in the front. The SI joint is the joint where the hip bones (specifically, the part called the ilium) hook up with either side of the sacrum. Although there isn't a ton of movement that goes on at this joint, there is a small amount of movement, and there are ligaments associated with the joint. I'm not sure what the doc means by "inflammatory disease," but it could be some sort of arthritis or perhaps ligament damage resulting in inflammation (kind of like a sprained ankle). The prednisone is a strong anti-inflammatory, so you were given that med to help with this problem.
3. L5-S1 disk. This is referring to the intervertebral disks, which are found in between each vertebra (bone) in your spine. They act like spacers, shock absorbers, and allow for bending your spine. The vertebrae are labelled depending on the area that they are found in and what position within that area they are in (numbered from top to bottom). The areas of the spine are cervical (neck), thoracic (chest), lumbar (lower back), and sacral (the sacrum). So the L5-S1 intervertebral disk is found between the fifth lumbar vertebra and the first sacral vertebra. Which means that the disk is the one just above the sacrum. This is in the same area as the SI joint, which is why I say that these two problems are possibly related. The disks can have several problems. They can just degrade and get compressed over time (which is generally referred to as osteoarthritis). The disk can also herniate (bulge outward). This creates more of a problem because it can push on nearby nerves. Because of reasons that don't really matter, you don't have to worry about the disk pushing on your spinal cord itself. But the disk can compress the nerves that are coming to and going from the spinal cord in that particular area. This can lead to numbness, tingling, pain, muscle weakness, or other neurological stuff. If a herniated disk is causing these problems, surgery may be needed to fix it. I'm not entirely sure which of these issues your doc is referring to when he says "pinched disk," but those are your basic possibilities.
So you basically had a whole bunch of fancy medical and anatomical terms thrown at you by the doctor, which can definitely sound scary. I hope that you were able to follow my attempts at explaining the terms. Not knowing the symptoms that led you to see the doctor, I can't really offer any advice or anything like that. If you are still not sure what your doctor was referring to with your diagnoses, you could call and ask for somebody to explain it to you in a way that you understand. It's easy for doctors to forget that most of their patients aren't familiar with the medical vocabulary. Remember that you always have the right to ask questions and insist that your doctor explain things to you in laymen's terms so you can understand what's going on.
I hope this helps a little. Best of luck to you!