A term coined by Rod Liddle in the Spectator in 2015, but which describes a phenomenon that is likely as old as time itself but has become more visible with the rise in prevalence of social media.
Virtue signalling, or, as I like to call it, conspicuous compassion, is a lazy form of pseudo-activism where people claim to support or cockwave about having once supported a cause, person, or thing that is generally approved of by the populace at large or which is likely to be approved of by those whom the signaller is trying to cosy up to. Often it is used to cynically deflect interest from other, more far-reaching or profound issues. The effect is to gain popularity and a reputation as a good person without much effort.
Whether or not the signaller believes in this cause is immaterial. If they do believe in it, then it costs them nothing and gains them much. If they don't believe in it, then any dishonesty aside, they can claim that they've seen the light or changed their minds. They get points for having abandoned their old, politically inconvenient ideas and hopefully critics are too overcome by the euphoria of having converted one of the Bad Guys that they don't see what's being done with the other hand.
That portion of the upper middle class left referred to in online parlance as "social justice warriors" are very good at virtue signalling. They have to be. They have to somehow reconcile their claimed support for progressive politics with their disdain and low expectations of the lower orders that they claim to represent, so they make a huge song and dance about gay marriage, transgender bogs, misogyny, intersectional feminism, etc. in the hope that nobody will notice that in a mob of well-heeled trustafarians moving into a once working class area, the locals are being priced out the market. Hackney in east London is a place where this goes on. While the hipsters colonise it with their ethos and liking for Doing the Right Thing writ large, few people notice that the working-class, deprived locals are getting kicked up the arse by the gentrification they bring with them. So here, virtue signalling is used to deflect attention away from gentrification.
It's not just the left that does it, though. David Cameron used the legalisation of gay marriage to deflect attention from a régime of benefit cuts that would be highly controversial. Tony Blair virtue-signalled about fox hunting to deflect the Labour left's attention away from his economic policies which weren't all that different from those of Margaret Thatcher.
Of course, outside of politics you can virtue signal as well. In business you can brag about "corporate social responsibility" to deflect attention from your dodgy business practices. Indeed, "corporate social responsibility" is almost always virtue signalling.