title: why we use " $ " in MongoDb aggregations?
body: <pre><code>$lookup : {
    from: &quot;categories&quot;,
    localField: &quot;_id&quot;,
    foreignField: &quot;userId&quot;,
    as: &quot;categories_details&quot;
  }
  </code></pre>
  <p>Why we used &quot;$&quot; as a prefix of lookup here?</p>