A geo-fence is just an imaginary line that marks a circle around the location. Then you need some kind of device that will report to smartthings when you cross that line.
There are a number of different devices that can do this. For some people, all they need is their regular smart phone.
Other people will find that a separate key fob type device is more reliable, but sometimes it’s the other way around. It just has to do with local architecture and geography and where the cell phone towers are located in lots of little details like that.
Yet another option is to get a Camera with face recognition. That can work well for the members of the household, but obviously doesn’t work for Airbnb or a rental cabin.
All of these options are just called “presence detection.” See the FAQ (this is a clickable link)