Steam's new library redesign (opens in new tab), which is still in beta, changes what's visible on individual game pages (beware scrolling down if you don't want to see a bunch of guides and community content with spoilers), adds organizational options like dynamic collections (opens in new tab), and generally moves toward a modern 'big portrait icon' style.
But as well as that, any achievement less than 10% of players have will now glow. This is visible only on said game's library page, so the effect won't show up if you check a game's individual achievement page or if you happen to have an achievement showcase on your profile, which is perfectly normal and not showing off. Oh, and the glow won't be animated if you selected "low performance mode" in the settings.
Anyway, it looks kind of nice I guess? And if you 100% a game you get a blue ribbon. You can see the glowing above in my achievements for Life is Strange. I was going to show you my achievements Divinity: Dragon Commander but nobody needs to know how many wives I collected in that game.