So basically if they get however many subscribers to their Youtube channel, they will boost the regular weekend XP event.
Every weekend they have this event:
05. Double IDR/XP Event
1. Log in during Saturday/Sunday between either 10:00 – 12:00 or 16:00 – 18:00 OGPTime (Pacific Standard Time), and receive 2x bonus Item Drop Rate, and Experience Points.
So with the subscribing event, when they reach the requested subscribers on YouTube, they will boost the regular 2x to 200%, 400%, or 600%