A release plan is used by product teams to visualize a detailed roadmap for features, improvements and bugs that will be built in the near future. It's time horizon is shorter term compared to most other roadmap types. In most cases it visualizes 6-12 weeks or 2-4 sprints.The main audience are product and development teams. It's less relevant to executives and external stakeholders because it provides a higher level of detail.
When deciding what items make it into your release plan you might want to consider customer value, strategic fit and the complexity of building it.