What is role hierarchy in Salesforce?

Role hierarchy refers to a set of roles in an organization and the relationship between different roles. Role hierarchy opens up vertical-level access to records.
Role hierarchy allows the user sitting in higher level have access of records owned by users having role lower in hierarchy. It helps to extend the OWD settings for different objects. It describes how roles connect to each other.
Users at any role level can view, edit, and report on all data that’s owned by or shared with users below them in their role hierarchy, unless your org’s sharing model for an object specifies otherwise.