Checking the Device Admin box helps Lookout help you.
In the Lookout Settings menu, you will notice an option labeled Device Admin. By enabling this capability you are providing the Lookout app with administrator privileges on your device. This in turn will allow the Premium Lock and Wipe features to protect you in the event that your device goes seriously missing. When you check the box you will be given a detailed explanation of what permissions this grants Lookout—including the ability to "modify or delete the contents of your SD card" which enables Wipe to work and "storage encryption" and "lock the screen," which enables Lock to prevent anyone from accessing the data on your phone without a four-digit code.
Note: You will not be able to uninstall Lookout while this protection is enabled and will need to take additional steps to remove the app.