This is just one section of an expansive 9/11 memorial at St. Michael's Cemetery. Also included are monuments dedicated to the PAPD (Port Authority Police), a PAPD bomb-detection dog, the NYPD, the FDNY (again), and Queens firefighters specifically, as well as a first responder memorial not related to 9/11.
Many of the names on these monuments (though not the monument pictured here) are accompanied by small medallions with QR codes printed on them. If you scan one of those codes, you are taken to a website with information and memories about the corresponding individual. (The company that provides this service charges $100 per medallion!) You might be able to spot some of the QR codes in the circles next to the names on the monuments linked above. Check out Sirius's memorial for a good example.