The ePrivacy Regulation lays down the rules for email, cookies and telemarketing. They are not specifically covered in the GDPR. The ePrivacy Regulation will replace the Dutch Telecommunications Act; the Cookie Act is part of it. Just like the GDPR, it is a regulation that has direct application.
This regulation provides for equal legislation throughout Europe; as a result, the opt-in rules for example will be the same throughout Europe. This does not mean that it is not important to prepare yourself properly and assess which business activities are affected by this legislation.
In particular the ePrivacy Regulation is about the use of personal data in online advertising, through services such as Google and Facebook (or other similar services). In broad terms this means that far more than before the specific consent of the end user is needed to work with personal data and data traceable to an individual.