https://www.polygon.com/guides/2018/6/25/17500674/pokemon-go-friend-list-trading-guide

Pokémon Go has always been a social game at heart, but it’s only as of the game’s most recent update that players can actually make friends with each other. The Friend List feature comes with various in-game benefits, from gift-giving to trading. It may seem light, but friendship is actually a big deal for the future of Pokémon Go.

You probably already have friends who play Pokémon Go, or whom you’ve actually met through playing the game. Niantic has now implemented a system in which you can actually interact with friends and track their activity in-game.