Simple Definition:

Collagen is a type of protein [a substance that our bodies need to grow and repair] that makes up a large part of our skin, tendons [connective tissues that attach muscles to bones], ligaments [connective tissues that connect bones to other bones], and bones. It helps to keep our skin elastic and firm, and provides structure and support to our body tissues.

Very Simple Definition:

Collagen is a special protein that helps our body stay strong and keeps our skin tight.