C++ Assignment to variable of summation between curly brackets

I'm currently implementing a class for an assignment for my C++ course, but there is one thing I don't fully grasp. As an example I'll use integers, to make this question more general. Let's say we have the following code:

#include <iostream>

int main{     
    int p = { 1 + 2 + 3 };
    int q = 1 + 2 + 3;
    std::cout << p << " - " << q << std::endl;
    return 0;
}

Why is the value for both p and q equal? What is the purpose of the curly brackets? Is there any difference in the way the program is executed or in terms of performance? I'm having a hard time finding the answer to these questions online so thought I'd ask it here.