It's usually done by faking the background. You place the image in a div (which is what he probably meant by layer :cringe: ), and then stretch the div.
Code:
#divbg {
width: 100%;
height: 100%;
left: 0px;
top: 0px;
position: absolute;
z-index: 0;   <------------Notice!
}

#maincontent
{
z-index:1; <--------------Notice!
position:absolute;
}
Then in your HTML,

Code:
<div id="divbg"><img src="abc.jpg" width="100%" height="100%"/></div>
<div id="maincontent">all text here</div>