It's a website/online service that lets you monitor and track a bunch of stuff about your Tesla, and aggregates information from all of its users about things like average efficiency, who is on what version of the software, etc. People get all excited when a new software version is spotted there.
You can learn more and sign up at
Home - TeslaFi.com. It costs $5/month (or $50/year), but there's a free trial. It requires your Tesla login credentials to generate an API key, and they claim they throw your credentials away after getting the API key. You can always revoke access through the API key by changing your Tesla password. You also have to beware of phantom drain because apparently by default the service will be polling your car often enough to keep it from going to sleep, and therefore wasting energy. They have instructions on how to configure it to avoid that, but it was a bit too complicated for me (and I'm a huge tech nerd). The iPhone app Stats gives me the same info, more neat features (like scheduled climate control, geofenced automatic Sentry Mode, and Siri shortcuts), so I'm just sticking with that for now.