The Misconception
Many developers think: "It's on GitHub, so I can use it however I want."
This is false.
Think of it like finding a book in a library. Just because you can read it doesn't mean you can photocopy it, sell it, or claim you wrote it. Code works the same way. The moment someone writes code, copyright law automatically protects it — no registration required, no copyright symbol needed. The author owns it.
Unless a piece of code explicitly contains a license file, you technically have zero rights to use it. It is copyrighted by the author. "No license" does not mean "public domain." It means "all rights reserved." You cannot legally copy it, modify it, or distribute it — even if it's sitting in a public GitHub repository.
When there is a license, it acts as a contract between the author and anyone who wants to use the code. The author says: "I allow you to use this code, IF you follow these rules." Some rules are easy ("Just keep my name on it"). Some rules are strict ("If you use this, your entire product must become free"). The license defines the terms; you either accept them or you don't use the code.
For an enterprise, violating these rules can lead to lawsuits, forced open-sourcing of proprietary products, and millions in dollars in damages. The Software Freedom Conservancy and individual copyright holders actively pursue violations. You must understand the two main families of licenses: Permissive and Copyleft.