The Product Manager represents the voice of the customer and business stakeholders (such as Business Owner and Business Sponsor) to the entire value stream or program (in a program-based organization). This includes writing the top-level descriptions of Capabilities that need to be created to deliver value to the customer and stakeholders.
The Product Manager also passes strategically-important development feedback to the business stakeholders in case they can adjust their capabilities plans to reduce costs/impacts, without hurting organizational goals.
Alternate or related roles:
Business Owner (especially for small organizations with small systems)
Business Product Owner (BPO)
Program Manager/Project Manager (adding PM duties highly discouraged)
Explore this role further in the Product Manager Reading Path.