StingRay is a brand name. The generic term is IMSI catcher. Basically a portable fake cell tower law enforcement can carry around that will capture the identities of every phone in the vicinity. They can also pretend to be 2G-only towers to trick phones to use an insecure protocol so more data can be captured.

Since it tracks everyone in the area, there's major privacy concerns about how they're used, and police have been caught repeatedly lying about their use. To the point that if the defense in a case discovered police had used a Stingray to identify their client and challenged it, the police would drop the case rather than letting anything about the Stingray enter the public record. https://arstechnica.com/tech-policy/2015/04/fbi-would-rather...