The 3-1-1 rule is a TSA rule applying to liquids, gels and creams that you are bringing with you on the plane, none can be more than 3.4oz or 100ml and all must fit in a single quart ziplock bag(clear). This bag must be removed from your carryon and be placed in a bin or on the x-ray belt for screening. If you forget to remove the ziplock bag with the liquids, you will be flagged by the x-ray TSA agent and your bag will be manually searched and waste a bit of your time.