To know what a macro does

Hi, I've been trying to sort a column that performs some calculations, but
postgres says this: ERROR: invalid attnum: -12851. I was searching on the
source code, and I guess the error araises around this macro:
/*
* Copy the given tuple into memory we control, and decrease availMem.
* Then call the common code.
*/
COPYTUP(state, &stup, (void *) slot);

So, I'd like to know what COPYTUP does?, what is availMem?, How can I tell
postgres it is a valid attnum?

Thanks for any help!!!!