Think of an instance where you want to create a database query class (or service)
You need to provide a connection to your database, this is what the query class uses to "make the necessary connections to the DB and executes queries successfully". Usually, a separate class handles the connection.
You can pass your connection class into your query class as a dependency, in other words, you inject the connection class (the dependency, a service or factory needed by other objects or services) into your query class which is the object that's dependent on another object.
public function conn()
//perform connections to the database
public function __construct(connection $connObj)
//we've just established a connection to the DB
The above code snippet is a better illustration of the DI concept. Think of the connection class as the "automobile object" and the query class as the "car object" for a "car class" analogy.
Watch out for the next part of the article that talks about how to properly inject objects, more on the single responsibility principle and how to build a DI container like Pimple.