Popularized by the wiz, Walt Disney. Plussing is a concept applied by product managers to make a good product great.
It is easy to apply.
You have a product in the market that customers are using (or you have just introduced what you believe is the MVP for your product).
You ‘plus’ by planning, scheduling and releasing additional features to the product. With the aim of making a good product great! Or converting a bad product to a good one.
To ‘plus’ you don’t thumb-suck what to add. Rather you rely on qualitative and quantitative measures to choose between the many possibly options to add to your product.
That’s where research methods like talking to the customers, reading the bug reports, interviewing colleagues in Ops, monitoring industry trends, monitoring macro-economic conditions etc., come to play.