AR Plus

AR Plus (short for Augmented Reality), stylized as AR+, is an enhanced version of Pokémon GO's augmented reality feature in which Pokémon appear anchored to a Trainer's real-world environment.

Background
AR+ leverages Apple's ARKit and Google's ARCore frameworks to build on the AR work already implemented in Pokémon GO. This makes AR mode look and feel more realistic, but also more challenging. Trainers can walk right up to a Pokémon for a chance at an Excellent Throw bonus or the perfect photo opportunity. Pokémon are aware of movement, so Trainers should proceed with caution, because moving toward a Pokémon too fast may scare it away, so approaching slowly is preferred.

Awareness
Pokémon no longer move around with the device, they stay in one place. Pokémon will run away if they become aware that a Trainer is getting close.

Trainers can sneak up close to earn an Expert Handler bonus, but they need to think twice before running right up to a Pokémon. An awareness meter will appear next to the Pokémon, and if the meter fills up, the Pokémon will flee. If it does, the Trainer can try tapping on the tall grass nearby, which may cause the Pokémon to reappear and give them an additional chance to catch it.

Expert Handler bonus
If a Pokémon is successfully captured while in close proximity, the Trainer will have a better chance at earning Great and Excellent Throw bonuses, as well as an all-new Expert Handler bonus that awards even more XP and Stardust.

Trivia

 * This feature was initially available only for iOS 11 on iPhone 6s and newer models.