It's the idea that people hold positions, or claim to hold such positions to signal (show off) how virtuous they are. For instance, say I don't have strong feelings about policy X. However, in my social circle policy X is seen as really bad. I then voice my disagreement with policy X, as a means of "signalling that I am virtuous". I would also potentially proclaim to hold strong disgust over those who agree with the policy, to distance myself from then.