In order to understand what "non-comedogenic" means, it may be helpful to break the word down. A comedo (or comedones, if more than one) is a type of pimple, or lesion that can form on your skin. Pimples differ according to their appearance and what causes them.
A comedo, the least severe form of acne, is the result of a clogged pore. Pores are the tiny openings in your skin that you can see when you look very closely, especially in a magnifying mirror. Inside those pores are hair follicles -- sacs beneath the skin that house the hair root -- and that's where pimples begin. When your body produces excess sebum, it can combine with dead skin cells and sometimes bacteria to plug up the pore. The result is a pimple -- a comedo. If the comedo is closed at the skin's surface, it's called a whitehead. When it's open at the skin's surface, and you can see the plugged follicle darkened by melanin buildup, it's called a blackhead.
Substances that cause your pores to become clogged and develop comedos are said to be comedogenic. Non-comedogenic products shouldn't clog your pores. Non-comedogenic is essentially “non-pore-clogging”. If you have combination, oily or sensitive skin, you should use "non-comedogenic" makeup and creams and cleansers.
However, there is one thing to keep in mind: there aren't really any regulations specifying what a company has to do to call its cleansers "non-comedogenic," and products labeled that way may not work for everyone. The next time you want to hide your inflamed and large pores with heavy foundation, stop and think a moment. Are you making the matters worse? Your main goal is not to get a reversed effect. With no guidelines or standards in place, even the thickest, greasiest moisturizer can claim it "won’t clog pores"! As a general rule, the thicker the product, the more likely it is to be pore-clogging. Instead, if you have oily or clog-prone skin, avoid products with a thick, creamy consistency. Look for products that have a liquid, gel, or extremely light serum texture, or a thin, water-based lotion consistency. Generally, products with thinner textures are less likely to clog pores or worsen breakouts.