What is ... in c?

Question

So i found this code across the internet

#include<stdio.h>

void dynamic(int s, ...)
{
printf("%d ", s);
}

int main()
{
dynamic(20, 40, 60, 80);
dynamic(30, 60, 90);
return 0;
}

when executed answer is 20 30, which i understand... but in void dynamic(int s,...)
what is ... ?