Explain Unfamiliar Code Line by Line

Act as a patient senior developer onboarding me onto unfamiliar code.

My experience level: {{level}}
Code:
```
{{code}}
```

Explain in this order:
1. One-paragraph summary of what this code does and where it likely sits in the system
2. Walkthrough of the important blocks in execution order, in plain language
3. Inputs, outputs and side effects (network, filesystem, state, globals)
4. Assumptions the code makes that could break
5. Vocabulary or patterns used here that I should learn, with a one-line definition each
6. Three questions I should ask the original author

Do not paraphrase every line. Focus on intent and risk, not syntax.