6Word on Web 'Catch up' button

Hi. I just opened Word online, and in the top-right corner, below my profile pic is the button shown below. It's greyed out, but when I opened another document, for a split second it active, but went greyed out again before I could click on it. I'm unable to replicate that. Also, I can't find anything about this on MS sites or the web in genral. Has anyone else seen this?