pci dss: What is two full-length key components? Requirement 3.5.3 of PCI DSS

PCI DSS 3.5.3 requirement

3.5.3 Store secret and private keys used to encrypt / decrypt cardholder data in one (or more) of the following forms at all times:
Like at least two key components of full length or key actions, according to a method accepted by the industry

Could you tell me what the full length key components are, what is the relationship with the secret and the private key store? LIKE a QSA, how to verify if the audited part meets the condition? Thank you very much ~