Use the fixed position to keep elements pinned to the viewport while you scroll.
Usually, you may want to use the fixed position with a navbar. When you want the navbar to stay fixed in position on your screen while you scroll through the page.
To add the fix positioning to a navbar:
You can of course choose any preset, or manually adjust the position of the navbar or any element you want to position this way.
Since, any fixed element is removed from the document flow, it will appear above auto-positioned elements. It may cover some part of your content. In that case, you may want to use some padding/margins on the content that the fixed element is covering.