Carter, a second year undrafted player out of Florida State, was with the Broncos in 2009 before being waived/injured before the regular season. Carter signed on to the Patriots' practice squad on September 7th. Despite standing at 5'9" and 175 pounds, Carter really impressed the coaching staff with his high motor and effort, winning multiple "practice player of the week" honors, indicated by him donning a black jersey in practice.
The Patriots signed Chevis Jackson just last week, so this move comes as a slight surprise. My feeling is that Jackson could be back, and the team promoted Carter, who has more experience having been here since September, in case cornerback Devin McCourty can not play tomorrow night. Therefore, I wouldn't rule out Jackson's return, once McCourty returns to health.
Still, this should be a good chance for Carter to prove his worth.
Update: For those interested in Carter, here's a draft video profile where you can see him in action, with the tip of the hat to NinjaZX6R