It’s “What might have been” hurricane season - it’s basically just an average of what the models predict a system will become. You’ll often see the most extreme intensity scenarios. For instance, in a WMHB season you may see more subtropical/tropical systems forming, or you may see more major hurricanes.

There’s also the popular CHP6 version of WMHB where the model with the worst-case scenario is used.

Not stupid I was wondering that too

It's perfectly fine to ask questions imo even if they sound stupid otherwise how else can one get answers but as Antoine said it stands for what-might have been