This explanation is long overdue since this term comes up so often, especially in the context of skincare. Collagen is a long, fibrous, and extremely strong protein located inside of your body's connective tissue. It is what keeps skin looking plump and elastic, and it is abundant in cartilage, bones, ligaments, tendons, and teeth.
As we age, our bodies begin to produce less collagen. That's why mature skin can appear saggy, wrinkled, and less resilient. Some topical products tout collagen as an ingredient, and while it can help keep your skin moisturized, it can't replace collagen that's been lost. However, collagen can be injected into isolated areas of the skin to create a firmer, smoother appearance.