The secret RSA key (N,d) consists of RSA modulus N – the product of two prime numbers p and q – and the secret exponent d. The secret RSA key must not be divulged and must be stored so that it is protected against third party access (for example PIN-protected on a smart card).
Public RSA key and secret RSA key are mathematically related so that with one key it is possible to cancel the function of the other one again.