Tummy time for a baby is when you place your baby on it's stomach during waking hours when the baby is awake.
It's important to always keep an eye on your baby though during the Tummy time and of course you should supervise your baby all the time or have someone supervise the baby at all times when it's awake.
Tummy time is important for preventing flat spots on the back of your babies head and it helps the baby to strengthen their muscles such as their neck and shoulder muscles and make those muscles stronger so they can learn and be able to sit up, crawl and even walk eventually.