“Plus value” is something that either directly gives you points, like territory, or can potentially give you points, like influence. “Minus value” is something that can take something away from your pocket, like cutting points, weak shapes and weak groups. If you have them, you can potentially lose something. The opponent has something there.
This article focusses on learning techniques based around Bloom’s taxonomy—remember, understand, apply, analyze, evaluate, create—and how they apply to Go.