Prior to React 16, components had to return either an HTML element or another React component as its content. This can restrict how you compose your application. For example, you might have a component that is responsible for generating an error message. You used to have to wrap these strings in HTML tags in order to be considered valid React component output. Now you can just return the string. Similarly, you can just return a list of strings or a list of elements.

The blog post introducing React 16 has more details on this new functionality: https://reactjs.org/blog/2017/09/26/react-v16.0.html.