Hover Card is a new OneDrive for Business File Office 365 feature. Basically now as soon as you hover your mouse over a file you will be able to see file details including access stats (number of views, who viewed and who modified).
In SharePoint Online document and page libraries, you’ll be able to see the number of views and number of colleagues who’ve viewed files and pages.
Hover Card also allows file owners to see if their colleagues have viewed their files. This feature is only available when files are stored in OneDrive for Business and is on by default. If you wish to hide the viewer list, as administrator, you can turn this feature off in your Admin centre settings before it becomes available to users across your organisation.
OneDrive for Business File Hover Card will be rolling out to Targeted Release organisations in the coming weeks and Microsoft anticipate completion of the rollout to Standard Release organizations in late-February.