I believe it indicates the thickness, or rather the thinness in this instance, of the condom in millimeters. For example, the .03 brand would be between .03mm and .039mm in thickness. Bear in mind the measurement could mean the average thickness throughout the entire condom or the thickness at a certain point. You'd have to read the back for that clarification. I'm not a sexpert but I created an account and researched this just to answer this question for you and me.