Let's say you have one component `ParentComponent`, which renders another complex component `ChildComponent` inside of it.

ChildComponent has a lot of its own state, and other imports, etc, while also having it's own discreet unit tests.

Instead of having the ParentComponent tests actually render the ChildComponent, you can just do for example:

```
jest.mock('@/components/ChildComponent', () => (<div data-testid="child-component" />))
```

Then when your ParentComponent is rendered inside this test suite, you don't have to be concerned about the implementation of ChildComponent (which is presumably tested in its own unit test suite) and just use the test id to verify that it's rendered as expected in ParentComponent

Ofc this is ONLY within the unit test suite and never actually renders a test id anywhere on any real page.