What is a polygraph test and how will it help in the Kolkata doctor rape-murder case?

The CBI has been authorized to perform a polygraph test on Sanjay Roy, the key suspect in the rape and murder case of a postgraduate doctor at Kolkata's RG Kar Medical College. The court deferred the next hearing to August 29. The polygraph test will help investigators check the consistency of Roy's statements and identify potential deception.