what is deep in vue watch code example

Example: vue watch deep

export default {
  name: 'ColorChange',
  props: {
    colors: {
      type: Array,
      required: true,
    },
  },
  watch: {
    colors: {
      // This will let Vue know to look inside the array
      deep: true,

      // We have to move our method to a handler field
      handler(value) {
        console.log('The list of colors has changed!', value);
      }
    }
  }
}