Basically, whenever your class needs an external resource, instead of newing one up itself, it should be given one to use instead.
Usually in the constructor.
If you have a class that accesses data from a database, it should be given an instance of DBReader to use rather than creating one itself. (making up class names here)
That way you can pass in a mocked DBReader that reads data from, say, an in-memory list rather than a database, so it's easy to unit test your class without setting up a whole database with dummy data first.
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