Synovial membranes line the inside of joints and produce fluid to lubricate joints and help bones to move smoothly over each other. In addition, synovial fluid helps to give the joint nutrients and to remove waste.
If a joint is injured, the synovial membranes become inflamed. The membranes produce more fluid than they should, stop absorbing waste effectively, and produce chemicals that cause even more inflammation.
This causes the joint to look swollen, and it will often feel warm. Your pony will probably be lame. If you notice these signs you need to call your vet immediately to get them to assess your pony.
Often the main treatment in the beginning is cold hosing and rest, but your vet will be able to tell you what you need to do. Sometimes your vet will need to prescribe medication and other times your pony may need treatment of the joint itself. It all depends on the cause, and how bad the damage is!