I have no idea

Like posting a bunch of photos that go against a streams policy
One LGBTQ stream got raided with a ton of burning pride flags for example

Essentially you spam images that go against the point of the stream, flag a lot of comments and images, make it an all together miserable experience for the users of those streams

Basically, stream raiding is posting images that violate stream rules (and potentially ToS) and flagging images/comments for no reason when stream rules/ToS are not being violated.