https://www.gamespot.com/articles/new-pokemon-go-update-out-now-heres-what-it-does/1100-6455790/

In addition to Pokemon Go's second Christmas event, a new update for the title is now live. The update--version 1.57.5 for iOS and 0.87.5 for Android--isn't nearly as large as the one that introduced Gen 3 Pokemon and weather recently, but it still adds a big new feature to the popular mobile game.

The centerpiece of the update is Pokemon Go's new AR+ mode. As its name suggests, this is an enhanced version of the game's existing AR feature, tailored around Apple's ARKit augmented reality technology. In AR+ mode, Pokemon appear closer to their actual size, and they'll be aware of their surroundings, possibly fleeing if you startle them by making quick movements. However, if you can get close enough to a Pokemon without startling it, you'll get an "expert handler" bonus that rewards you with extra XP and Stardust when you capture the monster.