If it's been some time since you've updated your Chrome browser, you might see a banner at the top of Gmail's interface on February 8th. It'll contain a reminder to update to Chrome version 55 from 53 and below, since the newer iteration comes with several big security updates. That banner will go away after you update -- unless you're still using Windows XP or Vista. Google stopped releasing Chrome updates for those two after version 49, since Microsoft no longer supports its older Windows platforms.