The Skull Island Topper tracks the highest level of Merasmus that the player helped to defeat.

On each level, the Merasmus grows stronger with faster health regeneration while hiding and increased range and damage to his hellfire attack.

The number shown, 2,147,483,647, is the largest value that a signed 32-bit integer field can hold. This was most probably accomplished by using plugins to rapidly spawn Merasmus, setting his HP to be very low and killing him immediately.