What is a tendon synovectomy surgery?

Let's say you have a nagging tendon injury that's really been painful. It's been bugging you a lot when you run. 

You have had persistent pain either on the side of the ankle that just won't seem to go away, even if you run less.  

You see the doctor and she says, “Look, we tried everything. We should do surgery. Your tendon is not really torn, but we should do this surgical procedure called a "synovectomy" to get it to calm down so it will finally stop hurting.”
 
What is a tendon synovectomy surgery? 
 
Well, that's what we're talking about today on the Doc On The Run Podcast.