What is the purpose of the program given below?

#define ledPin $13$

unsigned long previousMillis;

unsigned long interval $=1000$;

int ledState $=$ LOW;

void setup$()\{$

previousMillis $=$ millis$()$;

pinMode$($ledPin$,$ OUTPUT$)$;

$\}$

void loop$()\{$

unsigned long currentMillis $=$ millis$()$;

if $($currentMillis $-$ previousMillis $>=$ interval$)\{$

previousMillis $=$ previousMillis $+$ interval;

if $($ledState $==$ LOW$)\{$

ledState $=$ HIGH;

$\}$ else $\{$

ledState $=$ LOW;

$\}$

digitalWrite$($ledPin$,$ ledState$)$;

$\}$

$\}$