This tab is displayed only when one of the bulk keys described here is used or when the following key is added to the AppSettings section of your web.config file:
<add key="CMSDebugMacros" value="true"/>
On the Debug -> Macros tab, you can see which macros were recently resolved. For each macro, its exact expression, result (1 - resolved successfully, 0 - not resolved) and context from which it was called is logged. This is useful if you want to analyze how macros were processed.
The log can be cleared using the Clear macro log button.