Virtue signalling is when someone expresses a view or does something specifically to let other people know (consciously or subconsciously) that they are moral, good, or “true allies”.
For example, a quick social media post about an important topic, frequently expressing your support for a cause verbally, changing your profile picture to represent a movement, and so on. These can all be forms of virtue signalling if they do not involve concrete action to help people or change behaviour.