Virtue signalling is a term to describe people that are doing harm to a cause or providing no benefit to it, while also aiming to maximize the admiration they receive from others.
That is at least how it's meant to be used. tptacek isn't wrong to point out that the Right generally uses it to denigrate people that are trying to do good just because there is a little 'show' with their 'action'. This is evil because people shouldn't be dissuaded from doing good things. In fact they should be celebrated. The Right have pretty much completely missed the point of talking about signalling, which is credible/costly vs. cheap.