This ensures the 2012 first round pick's current deal will run through the 2016 season. Note that Cox was already under contract for the 2015 season.
The fifth-year option is projected to be worth about $7.799 million in 2016. The money is not guaranteed, except for injury.
This option does not prevent the Eagles and Cox from working out a long-term deal in the meantime. Reports indicate that both sides have not talked about a new contract yet, but it's expected to happen at some point.
Cox's name has come up in some Marcus Mariota trade rumors this offseason. For those wondering, the Eagles picking up the fifth-year option has no real bearing on whether the team will trade him. As noted earlier, the fifth year salary is not guaranteed. It's still "possible" (though not necessarily probable) that the Eagles could trade Cox.
In any case, Cox has been a great player for the Eagles. He's only 24 and already he's a monster at the 3-4 defensive end position.
UPDATE: Eagles officially confirm the news.