NOYB: "None of Your Business", it's the site being linked. They're a customer rights activist group led by Max Schrems whose self-declared job appears to be "drag every business in the world to the attention of the DPAs to make sure they're both compliant and don't do any funny business with the GDPR".
DPA: Data Protection Authority. They're local to each EU country (including those that aren't full Member States but do follow EU laws) and it's their job to enforce the GDPR at a country level. Usually they're a government department. The one you hear the most in the context of GAFAM enforcement is the Irish one, which is infamously underequipped and overburdened because GAFAM has lobbied the Irish government to not take enforcement seriously so all GAFAM companies can put their EU headquarters there (which permits them to only have to comply with the Irish DPA, something called the "one-stop shop" solution.)
CJEU: Court of Justice of the European Union. Think SCOTUS but for the EU. While technically the CJEU doesn't operate on the concept of precedent (its main job is enforcing that Member States follow EU laws and directives correctly), their interpretations of EU law are the final interpretation of those laws. Usually once a case has gone through a DPA and isn't resolved satisfactorily (or isn't complied with), it ends up before CJEU. GAFAM has dragged basically all enforcement cases before CJEU to drag out the process.
GAFAM: An abbreviation referring to the five largest American tech giants: Google, Apple, Facebook, Amazon, Microsoft. Usually is used as a shorthand for the "big tech" side of the IT industry.