Just a guess , but if you have say 4 different forums. News , Events , Duels , and Fort Battles. Maybe it allows you to move lets say Fort battles to the top of that forums list instead of being the 4th in line.
You got that right Dubb777. You can have different forums (or categories) however you want to call them that are lined up vertically left to right above the threads area. Those button allow you to rearrange them.