This attribute declares a macro. Think of macros as library of small templates which can be reused in any other template.

            <div metal:define-macro="main_menu"><ul><li><a href="/">home</a></li><li><a href="/products">products</a></li><li><a href="/contact">contact</a></li></ul><div>
    Last modified:
    <span tal:content="mdate">page modification date</span></div></div>

          

Macros inherit all variables from their caller. In the above example, the variable ' mdate' depends on the template that calls the macro.