Updated: Apr 1, 2019
The Explorer Window
This is a visual representation of everything you have or create in your game project. Starting at the top you will see Workspace. When you add a model to your game, or create a part you will see it appear in the Workspace. While you work with Roblox Studio you will constantly be working in the explorer window. It is usually place on the right of the screen, however you are free to move it somewhere else as you are with all window. Regardless of your setup, make this window a permanent addition.
The Properties Window works hand in hand with the Explorer Window and usually sits below it. When you interact with something in the Explorer Window by clicking on it the properties window is like a magnifying glass letting you look at all the details about the thing you clicked on. An essential window to have open while you create your masterpiece.
The Output Window
The output window sits at the bottom of the screen and displays actions you take, such as saving your game, error messages and when you choose to output from your code. Great for checking if parts of your code are working properly, or to simply say "Hello World!"