A security clearance is a status granted to individuals allowing them access to classified information (state or organizational secrets) or to restricted areas, after completion of a thorough background check.
The term "security clearance" is also sometimes used in private organizations that have a formal process to vet employees for access to sensitive information.
No individual is supposed to be granted automatic access to classified information solely because of rank, position, or a security clearance.
The BPSS is the entry-level National Security Clearance, and both CTC and EBS are effectively enhancements to the BPSS, with CTC relating to checking for susceptibility to extremist persuasion, and EBS relating to checking for susceptibility to espionage persuasion, the latter being needed for supervised access to SECRET material.
Those with National Security Clearance are commonly required to sign a statement to the effect that they agree to abide by the restrictions of the Official Secrets Act (OSA).