A skeleton or framework basicaly just sets the max width of a page and inserts collumns in that width.
For example you can have four collumns each 240px wide ( 4 x 240 = 960). This means that you can have one, two, three, four ( and possibly more) collumns which makes it super easy to customize a page with simple html coding with some css backbone.
For example: (simplified coding)
<div class="col960"> text here </div>
<div class="col720">text here</div> <div class="col240"> text here </div>
This would create a single collumn and beneath it a 3/4 and 1/4 collumn.
Needless to say this is a really easy and practical way of dividing a webpage into pieces
moreover it offers a simple way of making numerous pages on your website each with another collumn layout.
I agree that it might seem abstract but if you pay attention to premium web templates out there you will most likely see that 99.9 percent uses such a skeleton!
Hope I've helped ^_^
kind regards