When you implement position: absolute on an element,
- the element will be taken out of the document flow. The original position of the element will be given to the other elements in the document flow.
- the element will be positioned in reference to the next parent element that has been positioned as well. For example if the element has a parent element on which position: relative, absolute or fixed has been implemented, that element will be taken as the reference point. If such element does not exist in the document, the reference point is going to be the root element html. The root element html graphically and practically in this case mean the edge of the viewport.
- you, as the developer, are responsible for the positioning of the element and make sure that won't block or overlap any other elements that are in the normal document flow or otherwise.
If you'd like to see and example and play with the code, please go to w3schools.com.