Content under the header

HTML

<header id="headerBar">
  HEADER CONTENT
</header>

<p>HELLO WORLD !!!</p>

CSS

#headerBar {
    background-color: #2ba981;
    color: white;
    height: 80px;
    position: fixed;
    top: 0;
    width: 100%;
    z-index: 9999;
}