-r, --recursive
This tells rsync to copy directories recursively. See also --dirs (-d).

Beginning with rsync 3.0.0, the recursive algorithm used is now an incremental
scan that uses much less memory than before and begins the transfer after
the scanning of the first few directories have been completed. This incremental
scan only affects our recursion algorithm, and does not change a non-recursive
transfer. It is also only possible when both ends of the transfer are at least version 3.0.0.