I think the official TSA website gives quite some information. (TSA is the Transportation Security Administration)
Fro the FAQ on that same website
Q. What is TSA Pre✓™?
A. TSA Pre✓™ is one of several intelligence-driven, risk-based initiatives helping TSA move away from a one-size-fits-all model to provide the most effective security in the most efficient way. The use of risk-based initiatives will allow TSA to maintain its high security standards and improve the passenger experience while ensuring as many travelers as possible experience the benefit of expedited screening through the TSA Pre✓™ lanes.
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)
Q. How will I know that I have been cleared for TSA Pre✓™?
A. Travelers who have been cleared for TSA Pre✓™ will have information embedded in the barcode of their boarding pass. Once the boarding pass is scanned at the checkpoint, the passenger may be referred to a TSA Pre✓™ lane. Many participating airlines are printing a TSA Pre✓™ indicator directly on the boarding pass.