A Microsoft file hosting and synchronization service.
I've just been investigating this myself. A report in a school of the context menu missing the OneDrive additions.
Right-click on the OneDrive icon in the task tray and 'View online'. This launched a browser but I needed to auth with my MS account. Once I signed in I returned to File Explorer and the context menus returned.
So my line of investigation is now whether this is related to those above saying 'you are not signed into OneDrive'. I can see the client is signed in but the browser was not.