There are numerous definitions of portfolio vs project management. Here is how I think about the distinction.
My organization recently launched a project management office (PMO) and we are learning something new every day. The right side of the diagram is about project execution: the core set of skills and discipline an organization needs to deliver projects well. The left side of the diagram is about project selection and prioritization: the mechanism by which the right projects appear on the doorstep of the PMO.
Portfolio management unites the two by ensuring that the "right" projects are selected and then those projects are delivered "well". Portfolio management presupposes ongoing assessment of every project from suggestion to inception and then to completion. The concept is simple. Is it easy to achieve? Not by any means.