Save relative.html as absolute.html.  Put this in the body to replace the existing div elements:

<div id="container">
    <p>I am a paragraph.</p>
    <div id="contained">
    </div>
    <p>This is an excuse for some text.</p>
</div>

and this in the style sheet to replace the existing styles:

#container {
    position:relative;
    height:120px;
    width:300px;
    background-color:green;
    padding:0px;
    margin:0px;
    border:0px;
}
#contained {
    position:static;
    height:50px;
    width:200px;
    background-color:blue;
    padding:0px;
    margin:0px;
    border:0px;
}

Save and view.  The blue div appears inside the green one under the first paragraph.  Under it is the second paragraph as expected.

Now replace the first line of the styles for contained with these three lines (so they appear at the top just above the height line):

position:absolute;
top:10px;
left:50px;

The contained div is positioned inside it's parent div with the position being based on the top and left attributes.  It does not matter what the static position would have been the contained div is always going to be exactly where you tell it to be.  With relative positioning the contained div would be 10px lower than when it appeared just under the first paragraph.  With absolute positioning that 10px is taken from the position of the parent div ignoring where the contained div would have been and also all other elements inside the parent.

Both paragraphs ignore the absolutely positioned div as if it was not there.

Try moving the second div around by changing the top and left positions.  Absolute positioning allows you to place an element exactly where you want it.  For it to work the parent element (the container) needs to use relative positioning (or sticky positioning but that comes soon).

Unlike relative positioning, absolute positioning the element is actually at that specific location but it does not affect any other items. 

Absolute positioning is not ideal for most uses (it is not adaptable to different displays) but it can come in useful so remember it.