Note how DI's managed the singleton scope of the sayer for us. If we pulled out another User from the injector, it would be a new instance with the same instance to Sayer
Binders
A binder is analogous to a Guice Module. But, seeing how "module" means something in the ol' Node.js world, they're called binders here.
Sometimes your dependencies get more complicated. Like, you may have a MapView that's just an in-memory mock for your test, but uses Google Maps in the UI, and you're working on porting them to Leaflet. You can control this kind of malarkey with DI pretty well.
Providers, complex scopes. I'm sure plenty of other small weird things.
Good Ideas
The best DI apps have one call to injector.getInstance. Passing the injector around or pulling many instances out of it is likely a bad sign. Try to express hairy sections as classes that have some number of dependencies and let DI figure it out for you.