Synovitis is a chronic and painful arthritic condition as Williams can probably attest.  It is an inflammation of the lining of the joints or synovia.

shoulder 1

Image by TheNak via Flickr

Deron Williams out of next couple of games for the Nets.  He received platelet-rich plasma treatments which will require time off to heal properly.

Deron Williams will miss the Brooklyn Nets' next two games after receiving treatment for synovitis on both of his ankles, the team announced via Twitter on Monday. He received platelet-rich plasma treatments on the ankles, and as a result will miss a game against the Indiana Pacers on Monday and a Wednesday home game against the Denver Nuggets leading into the All-Star break….More at Deron Williams injury: Nets guard to miss two games after ankle treatment – SB Nation

He hasn't been playing up to par and has stated that rest has not really helped the injury.