the array Initialize in C++

#include <iostream>
using namespace std;

int main() {
    int arr[10] = {};
    for(auto element : arr)
    {
        cout<<element<<" ";
    }
    cout<<endl;
}

if i write int arr[10] = {}, the elements in arr are all 0. but if i just wrtie int arr[10], the elements in arr are random. So i am confused about int arr[10] = {}, i just declare a array int arr[10], but i don't give it any value, just a {}.