"Act/React" means each scene has a power struggle in it (can be on a very small scale up to a very large scale for comic book/action movies) and each main character is taking action and then reacting, constantly, in each scene and throughout the story. So it's important for your character(s) to have something and someone to react to. this not only holds audience interest but helps the character develop/learn/grow (or get worse, depending on what kind of arc you're doing).
Some movies pull this off with very few characters, and even in scenes where it's just the protagonist on their own there's still something for them to act and react to. Think about "INTO THE WILD" (which if you haven't seen, you must), the character has some long stretches where he's just totally on his own but he is doing things, reacting against or to the environment around him. In that movie, the environment is a major character, so he reacts to it instead of to a person in a number of scenes.
Act... react. Act...react. Like a two-step waltz, the dance of power each character goes through in every scene in every moment of your script.