C2 Proficiency is one of the Cambridge English Qualifications. It is their highest-level qualification – proof that you are a highly competent speaker of English.
A Cambridge English: Proficiency (CPE) qualification shows the world that you have mastered English to an exceptional level. It proves you can communicate with the fluency and sophistication of a highly competent English speaker.
Preparing for and passing the exam means you have the level of English that’s needed to study or work in a very senior professional or academic environment, for example on a postgraduate or PhD programme.