A security clearance is a status granted (approved) to individuals allowing them access to classified information, e.g. state secrets. It is something like permission or a license. The term "security clearance" is also sometimes used in private organizations that have a formal process of selecting employees to access "sensitive information".
A clearance by itself is normally not sufficient to gain access; the organization must determine that the cleared person "needs to know" the information. No one is supposed to be granted access to classified information only because of rank, position, or a security clearance.