Welcome to DC.
It just means to mix the butter and sugar together until they're fully blended, and there are no visible butter lumps or sugar crystals, it's all smooth. Beat the heck out of them. Room temperature butter works better than cold butter. You can blend the sugar and butter by hand, using a fork or a whisk, but it's easier with an electric hand mixer or stand mixer.
She who dies with the most toys, wins.