Chrome OS is a lightweight operating system from Google for devices such as laptops or tablets. In the beginning it was not popular because of the lack of application support, but after Google integrated the Android code and gave support to the Play Store, the use of the Chrome OS platform increased. Recently, Google has come with a new application store, the App Mall.
If you're wondering why Chrome OS needs to have another app store? Previously, users had to download from two stores, the Chrome Web Store and the Play Store. To make it easier for users, Google combines Android and web applications supported on Chrome OS in one simple store called App Mall.
Chrome OS still supports the use of Linux applications, but for these applications they are not offered on the App Mall and users have to go through the download and installation process as usual.