what does the signal ">" in the css selector "#aaa > .bbb" mean?

It means "child selector". The code says "affect the direct children of an element with ID='aaa', where the child's class is 'bbb'."

Internet Explorer 6 does not support this selector (Opera, Firefox, etc do), so it can be used as a way to avoid bugs in IE6's CSS support.

A complete reference of all the selectors in CSS can be found here: http://www.w3.org/TR/CSS21/selector.html

IE6 only fully supports type, descendant, class, and ID selectors.

-Ryan / Kardax