responsiveness using tailwing @apply code example

Example: what is md in tailwind css

// tailwind.config.js
module.exports = {
  theme: {
    screens: {
      'sm': '500px',
      'md': [
        // Sidebar appears at 768px, so revert to `sm:` styles between 768px
        // and 868px, after which the main content area is wide enough again to
        // apply the `md:` styles.
        {'min': '668px', 'max': '767px'},
        {'min': '868px'}
      ],
      'lg': '1100px',
      'xl': '1400px',
    }
  }
}