In application development, we try to create independent modules so that we can reuse code in future projects. But, it's difficult to create completely independent modules which provide useful functionality; their dependencies can cause maintenance nightmares unless they are managed properly. This is where Dependency Injection comes in handy, as it gives us the ability to inject the dependencies our code needs to function properly without hard coding them into the modules.
They start with a look at the problem with working with "concerete dependencies", ones that are hard-coded into your classes making them not only hard to test but potentially difficult to maintain. They include an example of this (a "SocialFeeds" class and friends) and then one of two ways to fix the situation. They start with using constructor-based injection, injecting the Twitter service into the main feeds object. They also talk about another method - setter-based injection - where the objects are injected via specific methods on the object.
As a third alternative, though, they get to using Pimple to manage the objects, making it easier to inject just the one resource into your classes and extract the objects you need from there. There's also a bit of "advanced" usage of Pimple showing the use of the "share" and "extend" methods.