When a chicken sits on an egg, it incubates it. To incubate means to keep something safe and warm so that it can grow.
The word can be used metaphorically to mean to keep something safe in order for it to have time to grow. You incubate a plan or an idea before bringing it into the world, or, metaphorically speaking, hatching it. Premature or particularly small babies are placed into an incubator when they are born. This warm, clean environment helps them to grow.