What does "cache-friendly code" mean and how does it affect C++ performance?

Hi! I'm learning C++ and came across the term "cache-friendly". I often see in performance discussions that code should be cache-friendly, but I don't fully understand what this means in practice. I have a simple example. I'm working with 1000×1000 matrices (int). Here are two code variants for summing them: Variant 1: by rows ```cpp int sum = 0; for (int i = 0; i < 1000; ++i) for (int j = 0; j < 1000; ++j) sum += matrix[i][j]; ``` Variant 2: by columns ```cpp int sum = 0; for (int j = 0; j < 1000; ++j) for (int i = 0; i < 1000; ++i) sum += matrix[i][j]; ``` The first variant runs 5–10 times faster on my computer! I've read that this is related to caching, but why such a huge difference? How do you even determine whether code is cache-friendly?