What does using array index like this [0,1,2] means?

How can an array have an index like [0,1,2]?
And why is [0,1,2]=[2]
Code:

int main(){
    int a[]={1,2,3,4,5};
    a[0,1,2]=10;
    for(int i=0;i<5;i++)
        printf("%d ",a[i]);
    return 0;
}

Output:
1 2 10 4 5

>Solution :

The comma operator (,) evaluates both expressions and returns the second one (see, e.g., this explanation). I.e., 0,1,2 will evaluate to 2, so a[0,1,2]=10 will result in a[2]=10, which explains the output you get.