This page discusses the usage of Portfolio for Jira live plans (any version from 2.0 to 2.27). If you're using the redesigned planning interface, see this page instead.
Portfolio for Jira dynamically loads your Jira issues into your plan, and suggests releases you can work with. The plan data is always up-to-date, allowing you to track the progress of your releases, and to know if these releases will be completed on time.
Plans also let you group project-specific releases into cross-project releases, which easily gives you a higher level view of your goals.
Check out some demo videos here.