Explain this code in detail:

```[LANGUAGE]
[PASTE CODE HERE]
```

Please explain:
1. **Overview** - What does this code accomplish?
2. **Line by Line** - Walk through the logic step by step
3. **Key Concepts** - What programming concepts are used?
4. **Flow** - How does data flow through this code?
5. **Potential Issues** - Any limitations or edge cases?

Adjust explanation depth for: [beginner/intermediate/advanced] developer