To explain shortly, an element who got the absolute position will be positioned in function of his parent element or the global page bloc with the coordinate 0 0 (Top Left). It will also be removed from the HTML flow such as the fixed property.
For the relative position, it stays into the HTML flow and it is deviate from its place into the flow.
The best to realize is to do position tests and see what are the visual results. Also, if what explained the freeCodeCamp class didn’t work to make ou understand, try to search the position property on Google and find the explanation that works for you.