There are a few theories that explain how language is developed. The first is that children model the behavior that they see. What this means is that children will immolate the language skills they see in their caregivers. For imitation in language to occur a caregiver must interact with language with the child.
Reinforcement is also a leading theory as to how language develops. In this theory a caregiver will use language and play to interact with a child. When the child reciprocates with actions such as a smile or a word they parent reacts positively (e.g. laughing, cheering, or smiling). This will improve the odds that the child will repeat the behavior because they want the positive attention from the caregiver.