Anyone know what it is?
Yes, it's code not to be compiled for the Arduino Due and the like.
If you need more help, provide more information!
There is ARDUINO_ARCH_ESP32. That's a bit fragile in that the ESP32 part of that macro name is generated from the architecture folder of the package. Due to the installation process of the package it's fairly common for people to mess that up.
There is ESP32. That's a bit fragile because for some reason they defined it in a property that is intended to be overwritten by the user.
There may be some lower level macros defined in the toolchain that would be more reliable.