"To whip the mixture" means to vigorously beat or stir it quickly with a utensil, such as a whisk or an electric mixer, in order to incorporate air and create a smooth and airy texture. Full definition
Then attach the bowl to your electric mixer and whip the mixture on medium speed for 3 minutes, scraping sides occasionally. (biggirlssmallkitchen.com)
The chilling of the mousse is so important... and do not over whip the mixture or you are on your way to chocolate butter! (epicurious.com)