ChromeOS is essentially the Chrome browser with a bit of window-dressing. There's a desktop space that lets you choose your own wallpaper. There's a start button in the lower-left corner for launching apps. There's an app launcher shelf where you can pin your favorite apps. There's a tray in the lower-right corner that provides quick access to Wi-Fi, Bluetooth, volume and settings.
Despite these various accoutrements, nearly everything you do in ChromeOS is done inside the Chrome browser. When you launch an app, the default action is for it to open as a new tab in Chrome. If you are like me, then you have enough tabs open in Chrome at any given moment without your apps adding to that figure. Thankfully, ChromeOS has an easy setting to change this behavior.
For any of the apps you have pinned to the app launcher shelf in the lower-left corner, right-click on an app icon and choose Open as window. That's it!
Now, when you click to launch that app, it opens in its own window instead of as another tab in Chrome. Better yet, the window doesn't look like a Chrome window with the URL bar and the tabbed interface and makes it feel like the app opened in its own space outside of Chrome. Sure, it's still just a Chrome window but it makes ChromeOS feel more like a full OS. And the best part is that it helps you in your efforts to keep the number of open tabs under control.
For more, learn how a Chromebook differs from a laptop.