What is a "cache-friendly" code?

Such a good question:

What is the difference between "cache unfriendly code" and the "cache friendly" code?

As a developer, you might have heard of the term "cache". It's

There are only two hard things in Computer Science: cache invalidation and naming things.

-- Phil Karlton

Here are some good answers from Stack Overflow:

• Stack Overflow: What is a "cache-friendly" code?
• Stack Overflow: Why does the order of the loops affect performance when iterating over a 2D array?
• Stack Overflow: How does one write code that best utilizes the CPU cache to improve performance?
• Stack Overflow: How and when to align to cache line size?