title: What is the meaning of the "@include" in .css Files?
body: <p>I don't understand , what is the meaning of the @include tag in .css files.
  For Example :</p>
  
  <pre><code>.wrapper {
  display: flex;
  width: 100%;
  @include display(flex);
  @include flex-direction(column);
  @include align-items(center);
  @include justify-content(center);
  @include transition(all 2s linear);
  }
  </code></pre>