The basic network interface for all platforms are the syscalls read() and write(). Every modern OS provides its own syscall to help network servers transfer files as fast as possible. lighttpd chooses the best one for available on each platform. This option exists to override lighttpd's choice in the rare case something is not working with lighttpd's choice.
Comparison of performance with different network backends is available here
server.network-backend = "sendfile"
server.network-backend = "writev"
server.network-backend = "write"