In software enginering - dependency injection is the process of creating an instance (object) of a class A and providing this instance to class B so that the functionalities of class A can be use in class B.
In context of Angular - dependency injection is the process of creating an instance (object) of a Service and providing this instance to a component so that the functionalities of service can be use in the component. One of the ways to implement dependency injection is through the constructor method of the component.