This is what you're looking for:
Like others said, bottom: 0 and position: fixed is what you need.
width: 100% will stretch the div across the page.
When you add fixed to an element, it then becomes independent of all other elements, which makes it tricky to position. I added left: 50%; and transform: translate (-50%, 0); it helps center the element to the page.
Source: (Center aligning a fixed position div)
div.bottom-nav {
  background-color: #232323;
  width: 100%;
  height: 55px;
  line-height: 55px;
  text-align: center;
  position: fixed;
  bottom: 0;
  left: 50%;
  transform: translate(-50%, 0);
}
a {
  color: white;
}
<div class="bottom-nav"><a href=#>Click me</a>
</div>