What are React and React Native used for?

What are React and React Native used for?

React: a library to build user interfaces.

UI: this is where people interact with the software (so, UI is the visible part).
Think of a simple button as an UI element.
Using React, you can define how that button should behave initially, when hovered over, and when clicked.
You can apply similar concepts for other UI elements (a paragraph, form, comment section, social links, etc…)

React Native: Uses react library’s concepts to build UIs for smart phones.

This should give you some idea. Mosh explains these concepts very well.
Take his free tutorials on YouTube and if you like, you can buy the full course as well: Code with Mosh