what is md in tailwind css

1// tailwind.config.js
2module.exports = {
3  theme: {
4    screens: {
5      'sm': '500px',
6      'md': [
7        // Sidebar appears at 768px, so revert to `sm:` styles between 768px
8        // and 868px, after which the main content area is wide enough again to
9        // apply the `md:` styles.
10        {'min': '668px', 'max': '767px'},
11        {'min': '868px'}
12      ],
13      'lg': '1100px',
14      'xl': '1400px',
15    }
16  }
17}