A framework that defines which parts of security are managed by the cloud provider and which are the customer's responsibility. It varies by service model: in IaaS the customer manages more, in SaaS the provider manages almost everything.
A central topic in AZ-900 and CLF-C02. Baseline rule: the provider is always responsible for security "of" the cloud (hardware, physical network). The customer is always responsible for security "in" the cloud (data, identities, permissions).
Shared responsibility model is a concept that can show up on the exam. Check the full guide to see it in context.