A service that lets you share AWS resources (Transit Gateway, subnets, Route 53 Resolver rules, licenses) across accounts in an organization without duplicating the resource or setting up manual trust relationships.
In SAP-C02 it's used to avoid creating a Transit Gateway per account: one is created centrally in the network account and shared via RAM to the rest of the organization. Question: share a subnet across accounts to deploy resources in the same VPC → RAM.
AWS Resource Access Manager (RAM) is a concept that can show up on the exam. Check the full guide to see it in context.