According to Gartner, a cloud access security broker (CASB) is an on-premises or cloud-based security policy enforcement point that is placed between cloud service consumers and cloud service providers to combine and interject enterprise security policies as cloud-based resources are accessed. Think of the CASB as the sheriff that enforces the laws set by the cloud service administrators.
Organizations are increasingly turning to CASB vendors to address cloud service risks, enforce security policies, and comply with regulations, even when cloud services are beyond their perimeter and out of their direct control.