Update for the 2021 App
SmartThings has two different kinds of modes.
Security.modes are used with smartthings Home Monitor. The values are now Arm (away), Arm (Stay) and Disarmed.
Location.modes are used in automations as a “behavioral filter.“ When you first set up a smartthings account, the values are Home, Away, and Night. But you can add more location.modes if you want. For example, many people add Guest mode or Vacation mode or even Party mode. Your choice.
Location.modes don’t do anything by themselves. You have to create automations to set them and then act on those settings. 
Location.mode is not shown under the LIFE tab, but Security.mode is.
You have to drill down a couple of levels from the main menu to find location.mode, I’m not sure why, it’s useful information.