Lucky Pokemon and Lucky Friends are special features in Pokemon GO. They are signified by yellow and white circles and sparks.
Once a Pokemon is traded it will have a chance of becoming a Lucky Pokemon. Lucky Pokemon will have increased stats, which can be seen when appraising the Pokemon. Additionally, fewer candies and stardust will be required to power-up a Lucky Pokemon.
After becoming Best Friends, it’s possible to become Lucky Friends with another Trainer. As Lucky Friends, the next Pokemon traded between the Trainers will become lucky. There is a chance to become Lucky Friends once a day by sending a gift, trading Pokemon, or battling with friends.