"Raid" is something that started out on Twitch. On twitch, instead of just ending a live stream, a creator can choose to "raid" someone, usually a smaller creator, and send all of his watchers to this other creator's stream where they proceed to "take over" the chat, usually with some good-natured, in-joke message. It disrupts whatever kind of chat this person had going, but since everyone only says it once or twice it's not considered spam. Streaming etiquette says after you've participated in taking over, you stick around for a bit and follow the streamer if you like what you see. It's a great way to find new streamers.
Some people, in particular Markiplier with his kick-club--but I'm not sure if he was the first, have taken this one step further and started the concept of charity raids. Instead of setting up a charity stream of their own they gather up their fans in a live stream and go in search of people who are doing a charity stream. There always are a few at any given time, often with a very personal story behind it. A loved one might have been diagnosed with cancer for instance, and to do something about the feeling of helplessness they started a stream to raise a few hundred dollars for cancer research. When the creator finds a stream like that, they share the link in the chat, everyone swoops in on the stream, donates a couple of dollars and disrupts whatever they had going at that moment. The creator who does the raids gets their content by showing the utter chaos and the reactions, the charity gets a bunch of donations and the small streamer gets a lot of new exposure. It is the best form of wholesome trolling the internet invented and a lot of fun to watch. Everyone comes out of these things feeling really good about themselves.