Could some tell me how this code centers something, ie this header.

Code: margin: 0 auto;

I don't understand why this would center my image.
thanks for any help.

It works in conjunction with a width property (by the way, margin is a property too.) 0 refers to the top and bottom margins (so there are no margins above or below your element) and auto refers to the left-right margins (50% of the remaining space goes to the left of the element, and 50% to the right of the element.)

For example:

margin:  0px auto;
width:  50%;

means that the element will be 50% of the width of its container, with 25% margins on the left and right sides.

Thanks for that.

OK would the hole thing to site in the middle of the screen but tried this

margin:  auto auto;
width: 600px;
height: 300px;

What i'm i doing wrong.

What i'm i doing wrong.

Nothing, it just won't work at all

BTW what's a margin tag?

Would it not be best to put why it would not work.

Margin tag is for creating a margin around a item.

It won't work because you can't apply an auto value for the top and bottom margins.

Margin tag is for creating a margin around a item

No it isn't. There is no such thing as a margin tag.
It is either;

an attribute of a visual HTML element tag

or

A CSS property of a style that can be applied to visual HTML elements

Ok thank you will try something else to get it right.