CSCS stands for Certified Strength and Conditioning Specialist.  At least it does in the United States.  In the UK it means something else entirely.  This website has everything to do with the former, not the latter.

According to the National Strength and Conditioning Association (NSCA) website that manages the CSCS certification, a CSCS is:

Certified Strength and Conditioning Specialists (CSCSs) are professionals who apply scientific knowledge to train athletes for the primary goal of improving athletic performance. They conduct sport-specific testing sessions, design and implement safe and effective strength training and conditioning programs and provide guidance regarding nutrition and injury prevention. Recognizing that their area of expertise is separate and distinct, CSCSs consult with and refer athletes to other professionals when appropriate.

Essentially the certification confers a degree of authority.  You have demonstrated that you know the science that drives athletic performance, and are qualified to coach athletes.

A CSCS does not test many attributes to coaching and training that are vital to success, like people skills.