DI is one of the ways to invert the object creation process from your module to other code or entity. The term injection refers to the process of passing the dependent object into a software component.
Since DI is one of the ways to implement IoC, it relies on abstraction to set the dependency. The client object doesn't know which class will be used to provide functionality at compile time. The dependency will be resolved at runtime.
A dependent object does not directly call to the client object; instead, the client object will call a dependent object whenever required. It's similar to the Hollywood principle: Don't call us, we'll call you when we need to.
