Google Mobile SERP Tests Showing Frequently Mentioned Keywords

Some users are seeing mobile snippets with labels located below the title. Each label contains either a single or multiple words that are mentioned within the content of a page. For the most part, the keyword labels only appear in instances where they are a relevant reference point. This means they may not appear for pages where the topic is straight-forward and the title and description combination provides enough context.

Barry Schwartz did write about something similar showing on mobile in March of 2017.