Resource Groups in Microsoft Azure is used to structure all your apps, databases, services, VMs etc. into logical containers that you can manage at once.
But when you create new resources, the Azure Portal often makes a guess about which resource group to use and you end up placing your new Web App in a different group than e.g. your storage account or your database.
That wouldn't be an issue if you could just move a resource to another group, but so far that isn't possible to do using the Portal.
Since Resource Groups is a new thing in the latest portal, it's even worse they don't have the ability to move resources to other groups, given the fact that resources from the old management portal were migrated into Resources Groups based on type (Web, Storage, SQL) and location.