what memset does in c code example

Example 1: memset c

#include <stdio.h>
#include <string.h>

int main () {
   char str[50];

   strcpy(str,"This is string.h library function");
   puts(str);

   memset(str,'$',7);
   puts(str);
   
   return(0);
}

Example 2: memset c

This is string.h library function
$$$$$$$ string.h library function