title: what's the real purpose of 'ref' attribute?
body: <p>I'm really confused with the "ref" attribute of the input element. I've never heard it based on my knowledge and can't find some meaningful materials about it.The code is in vue.js offical documents.</p>
  
  <pre><code>&lt;currency-input v-model="price"&gt;&lt;/currency-input&gt;
  </code></pre>
  
  <p>This is code about the component:</p>
  
  <pre><code>Vue.component('currency-input', {
    template: `
      &lt;span&gt;
        $
        &lt;input
          ref="input"
          v-bind:value="value"
          v-on:input="updateValue($event.target.value)"&gt;
      &lt;/span&gt;
    `,
    props: ['value'],
  </code></pre>
  
  <p>what's the meaning of the value of ref attribute that equals to input?</p>