Loaders transform files from different languages or formats into modules that Webpack can understand. They run during the build process and transform files on a per-file basis.

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: ['style-loader', 'css-loader']
      },
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: ['@babel/preset-env']
          }
        }
      }
    ]
  }
};

Loaders are processed right-to-left (or bottom-to-top) in the use array.