What does Magento mean by Dependency Injection, and why is it different from "just use new"?

Magento's DI container is in Magento\Framework\ObjectManager. You never call new Vendor\Module\Foo() in production code — you type-hint the dependency in your constructor and the container auto-wires it, applying any preference, plugin chain, or virtualType from etc/di.xml. This is what lets a third-party module plug your service without changing your code. Two flavours: Constructor Injection (preferred, type-hinted, eager) and Method Injection via ObjectManager::create() (only legal in factories, repositories, and setup scripts). Calling ObjectManager::getInstance() in a controller is the Magento equivalent of a code smell — reviewers will fail the PR.