The watery stuff in your mouth is called saliva. Saliva helps you swallow, but it can also be used to prep a spit ball.
You might know saliva as spit — but officially, when you spit, you spit saliva. There are special glands inside your mouth that produce saliva, which helps you chew and swallow, and also contains enzymes that start breaking down your food so it can be digested. Saliva is a Latin word, meaning "spittle," but its origin beyond that is a mystery.