The phrase refers to a fundamental version of the Android 4.0 operating system, also known as Ice Cream Sandwich, intended for use on Android devices. It represents a baseline software platform without device-specific customizations or additions from manufacturers or carriers. As an example, a development board running the vanilla operating system prior to vendor modifications would be considered to be using this.
Its significance lies in its role as a standardized starting point for device development. It allows for greater consistency in application development and testing across different hardware platforms. Historically, it was a key release that unified the smartphone and tablet versions of the Android operating system, offering a more consistent user experience across form factors.