Portfolios are simply groups of commonly associated projects and ideas. For example, you can have a portfolio for all the projects in your department or perhaps a portfolio for just your crazy ideas.
The benefit of creating a portfolio is that you can choose what you want to consistently measure across all the projects and ideas within it. For example, you may want to record Total Sales or Capital Expenditure projections across all your projects.
Any project or idea added to your portfolio will require the data demanded by the portfolio metrics you've chosen.
To see the total performance of your portfolio and it's breakdown by project, select the Metrics tab on your portfolio.