Agile Portfolio Management is a tool for a group of teams that have too many major chunks of work in flight. Traditional Portfolio Management is centred around the idea that we fund projects and assign people (or fractions of these people) to work on the project. By contrast in an Agile world, we have a fixed number of stable teams, so we can only have one project or major chunk of work being worked on by any one team at a time. This is a form of Limiting Work In Progress per team.
To make this work, the organization needs to see that the funding model in an Agile world is tied to teams, and then the Portfolio Management is just about deciding which chunks of work to do next. It is the art of getting stakeholders to help make big-picture decisions about the Product Backlog with you.