What does this "for" do? How does it work?

It is a for each loop in C++, and iterates over the columns that you have in your (most likely) grid. It is called a range-based loop; since you get a range implicitly in it.

And once it gets the column, it resizes it to a specific height—also notice the reference sign. Without the pass-by-reference your operation will have no effect on the columns. A quick example I created to demonstrate is here,

void processprint() {
    int array[] = { 1, 2, 3, 4 };
    
    // Increment the values, without modifying actual elements. 
    for (int item : array) {
        item += 2;
    }
    
    for (int item : array) {
        std::cout << item << std::endl;
    }
}

This will print the same array, unaltered, however if you change the line,

void processprint() {
    int array[] = { 1, 2, 3, 4 };
    
    // Increment the values, change actual values.
    for (int& item : array) {
        item += 2;
    }
    
    for (int item : array) {
        std::cout << item << std::endl;
    }
}

This will now print incremented values. Check it out here, C++ Shell[^]

Range-based for loop (since C++11) - cppreference.com[^]