The Android 5.1.1 Lollipop System Image is a download that contains all the files needed to install a stock version of Android Lollipop on the Nexus 7 (2012), the Nexus 7 2013 (WiFi) and the Nexus 10.

Nexus 4 and Nexus 5 gets Android 5.1.1 update over-the-air (OTA), check out the guide to install the updates manually.