Why did my boarding pass have a TSA pre-check mark on it?
I think the official TSA website gives quite some information. (TSA is the Transportation Security Administration)
Fro the FAQ on that same website
In my understanding, it means that the American government trusts you more, which helps you to pass security quicker.
Also, there is always a notification on your boarding pass, which is what you found (from the same FAQ)
You specifically asked why your boarding pass had a Pre-Check indication. The short answer is that the operating airline sent your itinerary (PNR) to the TSA before your flight, and the TSA sent it back with Pre-Check clearance for that flight. The TSA clears certain flights for certain people for Pre-Check because one of these happened:
(There is also a TSA program called Managed Inclusion whereby when you get to the airport, even though your boarding pass does not say Pre-Check, the TSA randomly directs you to the Pre-Check lane, possibly because the regular lane was getting so backed up. But in that case, your boarding pass doesn't show Pre-Check, and you asked why your boarding pass did.)