The Injector is a pluggable strategy to any IoC container such as Spring to be able to create and dependency-inject objects of a certain type.
For example if you are using the Inversion Of Control With Smart Defaults pattern to minimize your XML dependency; Camel will automatically discover and use new endpoints when they are referenced via their URIs by using the META-INF/services settings. You could explicitly configure each Component and Endpoint in your Spring XML file; or let Camel find the defaults then use the Injector to create and inject any of its dependencies.
As an example; consider the JMS component. Rather than explicitly configuring the JMS component in Spring XML you can just provide a ConnectionFactory in your Spring XML which the Injector will use to properly configure the JmsComponent when it is instantiated.