Overrides are sections of code that can be used to modify certain parts of your journal. They are inserted into the Style Overrides box near the bottom of the Modify Journal page (http://www.insanejournal.com/modify.bml).
The only HTML elements that can be used in the *_HEAD overrides are those elements that are valid in the head of a HTML document. This is limited to <title>, <base>, <style>, <link>, and <meta>. All other elements are stripped. If your overrides are not changing the look of your journal, that is most likely the reason.
There is more information available on overrides, as well as tutorials for using overrides, in the following locations:
There are a number of ways to use the override system. The above links describe only some of these ways. Due to the potential diversity and complexity of all available overrides, it would be impossible to officially support them all.