MUI is the company behind Material UI, an open-source React component library. React is a JavaScript library for building user interfaces from components, and a component library supplies finished components so that a team does not design and code each one. Material UI implements Material Design, the design system published by Google.
A developer installs the npm packages @mui/material, @emotion/react and @emotion/styled and then uses components such as Button, Text Field, Table, Alert and Tooltip in the React code; the library has more than 40 of them. Each component is self-contained and injects only the styles it needs. A theme changes colors, fonts and other design tokens for the whole application, so the components can follow a brand instead of plain Material Design. The sx property and the MUI System utilities change the style of a single element.
Material UI is free and open source under the MIT license. MUI sells paid plans for MUI X, a separate set of advanced components such as a data grid, date pickers and charts.