I mean when you have a class which, instead of importing/including/requiring its dependencies directly, expects instances of various classes to be passed into its constructor for example. So the class cannot do its job unless you pass its dependencies into its constructor... This is an antipattern which unit tests tend to encourage because it allows you to decouple the unit logic from its dependencies and it makes it easy to inject mocks in the place of dependencies.

A unit test should only test a single class so that means you need to mock out all other classes which your class depends on. Mocking out dependencies in the test code is difficult or not feasible and that's why developers often resort to Dependency Injection in their source code but as mentioned before, it is an antipattern.

Also, I noticed that a lot of developers confuse unit tests with integration tests but the definition is quite clear: If your test covers more than one class without mocks, then it's an integration test. Integration testing does not necessarily mean end-to-end testing. It could just be a single class with its internal dependencies.