L'oreal Magic Perfecting Base is a makeup primer that is part of the L'oreal Studio Secrets™ line.
Primers are designed to camouflage any imperfections in the skin, creating a smooth, even base for your foundation. They usually accomplish this by using silicone-based polymers such as dimethicone, which is one of the main ingredients in this particular primer. 
According to the product description, this primer minimizes the appearance of fine lines and pores while leaving the skin soft to the touch. It can either be worn alone or under foundation.
The primer should be applied in the morning before putting on your makeup. It can either be applied directly to your bare skin, or you can apply a moisturizer first. If you do use a moisturizer, allow it to soak into your skin completely before applying the Magic Perfecting Base.
Once your skin is ready, apply a dot of the product to one part of your face, using your fingers to blend it into the skin with a gentle circular motion. Continue on to the next part of your face, repeating the process until all of the areas where you want it have been covered. Typically, this includes the area under the eyes and across the nose and cheeks. If you have an oily T-zone, you can also apply it to your forehead and chin to keep your skin matte and to help your makeup last all day. 
This video provides an in-depth review of the product along with showing how to apply it: