Pokemon Go raids are another daily occurrence in Pokemon Go nowadays, and if you want to get some of the rarer Pokemon, and some of their evolutions, this is a great way to do it. They appear atop Gyms, and the game will tell you what’s appeared and where. You’ll need a Raid Pass (you can use and collect one per day), but after that it’s just a case of wandering over to the relevant Gym as and when the Raid is live.
It’s worth noting though, that these are tricky. You’ll want to gather some friends to battle with you, or make sure there are other trainers also joining you for the fight when you get there. The difficulty level is marked in masks under the Raid, and while it’s possible to take down a level one or two solo, you’re going to have to enlist some fellow trainers for anything tougher than that.