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A general description is here.
Defining ‘R_INCLUDE_BOOLEAN_H’ to ‘0’ before including headers ‘R.h’ or ‘Rinternals.h’ (or any other header which includes ‘R_ext/Boolean.h’) stops the inclusion of header ‘R_ext/Boolean.h’ which `defines' constants ‘TRUE’, ‘FALSE’, ‘true’, ‘false’ and the type ‘bool’ which some package maintainers wish to avoid.
Note that the last three are keywords in C23 and C++11 so cannot be avoided entirely. However, with commonly-used compilers they can be masked by a macro of the same name, often with a warning.