Code questions may belong on one of several sites
- Broken or incomplete code belongs on Stack Overflow
- Working code that you want feedback on belongs on Code Review
- Code does not belong on Programmers unless it is supporting your question: while this is a conceptual site, it is about programming after all. But a question dealing only with a block of code would be closed as off-topic. Think of it like this: if your question is a thesis paper, code is like references. They help, but a thesis full of only references is no good.
That being said, "help me understand this code" is generally off-topic everywhere. Sometimes those question will slide by on Stack Overflow, but only if there is a suspected bug or other code malfunction (i.e. does not work as expected). I would search Stack Overflow's meta site and maybe ask on their meta first to be safe.