Google wants to help keep our browsers clutter-free. The company is testing out a way to remind you to close old tabs. It has been working since August, but now it's showing up more prominently. The "Suggest to close Tabs" feature is hidden behind a flag in chrome://flags, but some versions of Chrome (like the Canary build) have it enabled by default.
If enabled, the feature will analyze your usage and check which tabs are unused or "stale." It isn't clear how Google determines what constitutes a stale tab beyond general activity or interaction with it. The feature won't automatically close the tab for you. Instead, it will open a window and let you determine which tabs it should close. Google hasn't announced if or when it will release this feature to the general public.
Source: 9to5Google