The Android version that introduced screen pinning was Android 5.0 Lollipop.
Screen pinning lets a user lock the device to one app or task, temporarily preventing access to the home screen, other apps, and—depending on configuration—notifications. Android 5.0 added both the user-facing feature and an API that applications could use to activate task locking.
The feature is useful when handing a phone to someone else, letting a child use a game, displaying a menu or ticket, or operating a simple kiosk-style experience. Users typically enable it in security settings, select an app from the recent-apps view, and pin it. Unpinning can require the device PIN, pattern, or password.
Screen pinning is related to Android’s broader lock-task and device-owner capabilities, but the terms are not identical. Managed-device administrators can use lock-task mode for stronger kiosk controls, while ordinary users can use app pinning for a temporary restriction. Lollipop also introduced Material Design and replaced Dalvik with ART.