Just think of them as defining functions that take s expressions in and spit s expressions out before “normal” runtime evaluation and which hence only see built in symbols.
But then there are things like hygiene, performance and some tricky edge-cases.
And I couldn't find any standard (and simple) algorithm to implement macros (preferably written in something other than Scheme itself).
Still trying to wrap my head around.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.464...