I think it means if most workers give the same answers their hits are accepted, & the people who give different answers get their hits rejected.
With majority rules, a requester gives the same HIT to 3 different workers. If your answer doesn't match the other 2 worker's answers, you are rejected. Basically this means if you are working on the same set of HITs as someone click-spamming or entering nonsense, your work will be rejected no matter how correct you are. And -- most who use it will say in the instructions something along the lines of "Your work is being checked against other workers" and if it doesn't match you'll be rejected. Not all will say so though, so sometimes it's better to ask before you start. Search the forum for the requesters name, search TO, or email the requester. I've emailed them to let them know I won't work on them if it's majority rules and the requester had a "password" for me to put in the comments box of the HIT so I could do them w/o fear of rejection Only one requester did that, so don't expect it to be the norm, but it never hurts to contact them.