A service that provides dedicated, single-tenant hardware security modules (HSMs), validated to FIPS 140-2 Level 3, for generating and managing encryption keys with full customer control over the hardware.
In SAP-C02 it's compared to KMS: KMS is multi-tenant, managed by AWS, FIPS 140-2 Level 2; CloudHSM is single-tenant, offers more control and compliance (Level 3), but the customer manages the keys and the hardware is more expensive and complex to operate.
AWS CloudHSM is a concept that can show up on the exam. Check the full guide to see it in context.