In a situation where there exists mutually exclusive positions A and B, a concern troll is someone who supports A but professes to support B around genuine supporters of B. However, they express their "concerns" about aspects of position B in order to sow doubt and uncertainty amongst genuine supporters of B.
Whilst this does genuinely happen, the term is used by some paranoid people to effectively mean "anyone who does not agree entirely with the standard dogma of position B, thus must actually covertly support position A", when said person does actually support B, just only 99%.
Thus: disagreeing *at all* with the standard dogma of B will get you branded by such people as The Enemy, and you will get generally treated with hostility and suspicion and, at worst, forcibly ejected from the B-supporter group. Result: everyone stops thinking about the issues, and discussion descends into not arriving at logical and interesting places but who can most furiously support B.
1: "All Democrats should be hanged, drawn and quartered! Grrgrrrgrrrgr!"
2: "Uh.. that's a bit excessive. I mean, I don't agree with the Democrats, but maybe we shouldn't actually, like, KILL them.."
1: "CONCERN TROLL! CLEARLY YOU ARE 100% DEMOCRAT! BEGONE, SINNER!"
(everyone flings rotten fruit at 2 until he leaves