Transparency is a key prerequisite for accountability in the GDPR.  Itsmain focus is to inform data subjects upfront^[1] of the existence of the processing and its main characteristics.  Other information (e.g. about the data subject) is available on request.  Data subjects must also be informed of certain events, notably data breaches (in cases where the data subject is exposed to high risk). Transparency is supported by controllers designating a DPO, who acts as a single point of contact for concerns by data subjects. Under the GDPR, data subjects are empowered to be the main guardians of their own rights and freedoms. Evidently, transparency is a prerequisite for detecting and intervening in case of non-compliance.

^[1] Upfront in this context means that data subjects should be aware of the processing before it takes place.  It does not imply a certain method of providing information or exclude dynamic ways of providing the necessary information.