If you implement position: fixed; on an element, it'll be taken out of the document flow and it's original position will be given to the other elements in the document flow.
If you don't position it properly, the position: fixed; element might overlap or block some other elements in the document flow.
The element will be positioned according to a coordinate system relative to the viewport. So position: fixed; bottom: 0; right: 0; would send the element to the bottom right corner of the viewport.
Since position: fixed; is based on the view port, the element will not scroll with the page anymore.
If you want to see an example and play with some code related to position: fixed;, please go to w3schools.com.