Screens are the backbone of every prototype. Prototypes include multiple screens, and each screen can contain UI elements. A screen in Proto.io corresponds to a screen in your application’s prototype. Depending on the fidelity of your prototype (low to high) they may also include simple or more complex interactions based on gestures and touch events.
The canvas that appears at the center of the Proto.io Editor represents a screen and you are essentially designing on it. This metaphor is taken a step further with the Proto.io Player with which you preview your screen(s) on your selected device and experience all the interactions you added.