Rattlesnakes do not lay eggs. The young develop inside the female from eggs and are then born alive as small rattlesnakes. This type of reproduction is called being ovoviviparous.
Mammals, such as humans, also give birth to live young, but their mode of reproduction is called being viviparous. In mammals, the young grow inside the female and are nourished via a placenta that connects to the mother's blood circulatory system.
In ovoviviparous animals, such as rattlesnakes, fertilized eggs develop inside the female. Instead of being laid, the eggs are retained inside the female. The young are nourished from the yolk of the egg. After the young are sufficiently developed, they are expelled from the female's body.