Pokemon GO update adds improved AR features on iOS devices1
The new AR+ technology takes advantage of Apple's ARKit framework. So, players using devices running iOS 11 like the iPhone 6s and newer models will be able to experience the new AR+ in Pokemon GO.
Keep in mind though that Pokemon will now run away if they become aware of the players getting too close to them. However, you'll be able to sneak up close to them to earn an Expert Handler bonus, but you'll need to be extra cautious when trying that.
A new awareness meter will now show up next to the Pokemon, and when it's filled, the Pokemon will flee, so keep an eye on that meter. You get one more chance at catching that Pokemon that has just fled by tapping on the tall grass nearby, which might cause him to reappear, but that's not a given.
The Expert Handler bonus awards more XP and Stardust than usual. Overall, if you can catch a Pokemon while in close proximity using AR+, you'll have greater chances of earning better Throw bonuses.