React children are a unique concept in React where props are passed to a component by position. They can be passed as named props or objects, and their data structure changes depending on how many there are, becoming an array when multiple elements are provided. The `React.Children` module provides functionality to work with children more efficiently, such as converting them to arrays, counting the number of children, ensuring only one child exists, mapping over children without worrying about their type, and iterating over children using `forEach`. Understanding React children is crucial for creating reusable, flexible, and composable components.