Niantic released the latest content update for Pokemon Go on iOS and Android. The update brings a Buddy Pokemon system to the game, which allows users to earn extra Pokemon Candy by walking certain distances.
With this Buddy Pokemon feature, a user can select one of their Pokemon to be their trainer’s “buddy,” and they can earn Pokemon Candy for this Pokemon by walking the required distances. The distances can vary, depending on the Pokemon. For instance, players may need to walk three kilometers to earn an Oddish Candy and five kilometers to earn an Eevee Candy.

Users can select their buddy Pokemon by tapping the trainer icon, tapping the menu button and tapping “Buddy.”
A user can assign different Pokemon to be their “buddies” over time, but they will lose any progress toward their next Pokemon Candy when they switch. In addition to the new Buddy Pokemon feature, this update includes support for the Pokemon Go Plus, the device that allows users to receive alerts about in-game events without looking at their phones. For instance, the light on the Pokemon Go Plus will flash when a Pokemon is nearby.
Here are Niantic’s release notes about this update:

Implemented Buddy Pokemon: Trainers will now be able to choose one of their Pokemon to be their buddy. A Trainer can earn Candy for their Buddy Pokemon by walking a certain distance.
Made it easier to select smaller Pokemon on the screen.
Fixed an issue where Eggs...