The Wikipedia article seems good:

https://en.wikipedia.org/wiki/Virtue_signalling

The short version is, it's when someone acts in a "moral" fashion not because they truly believe it, but because they want others to see them as virtuous. Imagine someone attending a protest purely so that their friends can see them protesting, or donating to charity just so they can tell everyone that they donated.

One minor nitpick: in social media the criticism generally applies mostly to mere statements, especially statements of support or outrage over any given emotionally loaded issue.

Attending a protest or donating to charity is relatively risky and expensive in comparison to just feigning outrage, retweeting slogans or changing your avatar.

Yeah virtue signaling is mostly a Facebook/Twitter thing.

YouTube too. Just pick any Sargon of Akkad video at random, it's almost guaranteed to be about either "virtue signalling", "moral busybodies" (i.e. the people mostly engaging in virtue signalling) or "cultural Marxism".

Or if someone proclaims "support the troops!" yet doesn't donate to veterans efforts, or support simple policies toward that end.

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.

I've never heard the phrase, and I read the news every day.

The phrase is more common in social media than in the news. It's not exactly the kind of language you'd expect in a news article.

It's not a term you would often hear in the news.

Me neither.

I'd define it "When in Rome ...."

Personally I would never attach the word "virtue" to opinion/thought only actions.