It's called a CSS expression. Google can give you a better explanation, but in short it's a way to assign the result of a Javascript expression to a CSS attribute, rather than a static value.
EDIT: if you need a literal explanation of the code itself, it is stating:
if the body width is less than 782 then set the width to 780 else if the body width is greater than 1262 then set the width to 1260
width:expression(document.body.clientWidth < 782? "780px" : document.body.clientWidth > 1262? "1260px" : "auto");
I have never come across anything like in css what does it mean?
Thanks for your help?
Andy
EDIT: if you need a literal explanation of the code itself, it is stating:
if the body width is less than 782
then set the width to 780
else if the body width is greater than 1262
then set the width to 1260