whats the role of "#" in gcc

can someone explain whats the function of "#" in the gcc code ?

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int main(void)
{
	system_init();

#if USE_EIC == true
#  if USE_EIC_NMI == false
	configure_extint();
#  else
	configure_nmi();
#  endif
#endif

#if USE_INTERRUPTS == true
#  if USE_EIC == false
	configure_systick_handler();
#  else
#    if USE_EIC_NMI == false
	configure_eic_callback();
#    endif
#  endif

	system_interrupt_enable_global();

	while (true) {
		/* Do nothing - use interrupts */
	}
#else
#  if USE_EIC == false
	while (true) {
		update_led_state();
	}
#  else
	while (true) {
#    if USE_EIC_NMI == false
		if (extint_chan_is_detected(BUTTON_0_EIC_LINE)) {
			extint_chan_clear_detected(BUTTON_0_EIC_LINE);

			update_led_state();
		}
#    endif
	}
#  endif
#endif
}
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