A "whisk attachment" refers to a tool used in cooking appliances, typically a mixer, that is shaped like a whisk and helps mix ingredients thoroughly and make them fluffy or frothy. Full definition
Beat egg whites and a pinch of salt on high speed in the bowl of a stand mixer fitted with whisk attachment until soft peaks form. (epicurious.com)
Using an electric mixer on high speed (use whisk attachment if working with a stand mixer), beat eggs, lemon zest, and 1 cup plus 2 Tbsp. (epicurious.com)
With the wire whisk attachment on your mixer, at full speed beat the hot syrup slowly into the gelatin until mixture is very stiff, about 15 minutes. (thisishowicook.com)