A serum is a lightweight, fast-absorbing liquid skincare product that is applied to the skin before moisturizer. Serums contain high concentrations of active ingredients that penetrate deeply into the skin to target specific skin concerns, such as fine lines and wrinkles, hyperpigmentation, and lack of hydration. Serums come in different forms, including liquids, gels, and oils, and they can be formulated with a variety of ingredients, such as vitamin C, hyaluronic acid, and retinol. Serums are designed to work in conjunction with other skincare products to enhance the effectiveness of your routine. It's important to choose a serum that is appropriate for your skin type and skin concerns, and to apply it as directed for best results.