They scale with the highest-level character currently online to function, meaning that if the highest-level character is, for example, level 100, only monsters up to that level can have raids. As progression advances, more monsters will also have the possibility of having raids.
Whenever a raid occurs, a message will be broadcast to the entire server, like this , indicating which monster it is, so you can go there and kill it. To receive the reward, you will need to deal a minimum amount of damage. Once the raid boss is killed, you will be rewarded with Silver Raid Tokens. You will not receive additional tokens if you deal damage beyond the minimum amount.
Once a raid boss is killed, another raid cannot appear for some time.