Popular topics being discussed on the social network will appear in users' news feed. By clicking, they'll be able to see what more people – including those outside of their own network of connections – are saying about it.
For now, at least, it's strictly an experiment – the feature in a limited beta at the moment, being rolled out to a small percentage of mobile users in the US, and there's no guarantee it will ever see a wider release.
The feature doesn't come as much as a surprise, given Facebook introduced hashtags in June, but what is particularly interesting – and encouraging – is that the test is beginning with mobile. Despite the company's much-touted 'mobile first' strategy, new features have tended to be introduced to the desktop incarnation of Facebook before rolling out to mobile.