what is meaning of product 2a 3d in c

1meaning of product*= number in C language is 
2product = product * number

1#include<stdio.h> 
2
3int main(){
4    int a, n;
5    printf("Enter the number !\n");
6    scanf("%d",&n);
7int product=1;
8for(a=1;a<=n;a++){
9    
10product*=a;
11
12}
13
14printf("The factorial of %d is %d \n",n,product);
15
16    return 0;
17}
18