A "ruckus" is a loud & rowdy noise, commotion, disturbance, fight, etc. A "ruckus" ranges from being mild to medium in terms of severity. A "brawl" would be considered much more serious than a ruckus.
If a child was banging on pots and pans, you would say they were making a ruckus.
If the local sports team had won and there was a loud, boisterous, celebration in the pub, possibly with some chairs being knocked over and some glasses accidentally smashed, you would say there was a big ruckus in the pub.
If people were picketing outside a courthouse, and somebody hit someone else over the head with a placard, causing jostling and a fight to break out, you could say that the placard hitting person was the cause of the ruckus.
Records show the word "ruckus" has been in use since at least the late 1800s. Origins of the word are not completely clear, with the most popular cited theory being that "ruckus" is a combination of "rumpus" and "ruction". There's some musing about the possibility of undocumented influences from the word "raucous" also.