what do watchers watch for in vue code example

Example 1: vue watch props

new Vue({
  el: '#app',
  data: {
    text: 'Hello'
  },
  components: {
    'child' : {
      template: `<p>{{ myprop }}</p>`,
      props: ['myprop'],
      watch: { 
      	myprop: function(newVal, oldVal) { // watch it
          console.log('Prop changed: ', newVal, ' | was: ', oldVal)
        }
      }
    }
  }
});

Example 2: vue add watcher

vm.$watch('person.name.firstName', function(newValue, oldValue) {
	alert('First name changed from ' + oldValue + ' to ' + newValue + '!');
});