I’m not a doctor but I’ll try:
Dystrophy refers to a degeneration, and algodystrophy is a condition that occurs usually after a trauma in which pain and swelling, as well as discoloration of the skin, has no known root cause (seemingly). Theories suggest it has something to do with abnormal irritation of nerves and those nerves reacting with sending inappropriate impulses. That being said, I think medical science is still struggling with this.
It sometimes is also known by an old terms: reflex symathetic dystrophy and complex regional pain syndrome.
If you or someone you know has been diagnosed with this, it’s important to get to a specialist. Any condition with “dystrophy” suggests there is a risk of functional impairment if the body or affected area gets too weak.