This system variable holds your Google API Key (if you've ever entered one). It is saved on your computer, specific to your login account, and not saved with your models. It persists across sessions. It holds arbitrary text.
There is no built-in functionality associated with the system variable, other than persisting the value across sessions. But it is intended for the purpose of saving your Google API Key -- a form of credentials that allows you to use Google's API services such as Google maps. (In general, these web services require Analytica Enterprise or better). Use of these services may result in billing, although Google generally allows you to use a pretty generous amount of these services for free before they start billing. Analytica itself does not use any of these services, but models that you load and use might, and models or libraries that use this system variables enable you use them on your Google account.
To obtain your own API Key, visit: Get API Key on Google's website. Then open the object window for UserGoogleAPIKey by pressing Ctrl+F and searching on identifier for UserGoogleAPIKey. The Object window opens. Paste you key into the Definition -- don't put quotes around it.