Everette Brown finally found a home after the former first second round choice of the Carolina Panthers was released earlier this season. Though he was a bust with the Panthers, young players are rarely let go at that stage unless it clearly isn't working out (a la Aaron Maybin of the Bills). Just look at the Indianapolis Colts still hanging on to Jerry Hughes for any sign of proof that you sometimes hold on just to save face with a busted draft choice.
As for what the Chargers are getting, it's hard to tell what they're getting. The Panthers play a 4-3 and now Brown becomes a linebacker. He had 23 sacks at Florida State and 46 tackles for loss, so it's clear he knows how to get past the offensive line. Whether or not he has the moves to do so at the next level is likely what's held him back, but you can never have enough of a pass rush, so this is a nice gamble for the Chargers.