Claimed capabilities of the implants range from telepresence to mind control to biotelemetry (the latter akin to humans tagging wild animals for study).
According to Peter Rogerson writing in Magonia magazine, the concept of alien implants can be traced to a March 1957 Long John Nebel radio show interview with UFOlogist John Robinson where Robinson recounted a neighbor's claim of being kidnapped by aliens in 1938 and kept subdued by "small earphones" placed behind his ears.
[2] According to skeptical investigator Joe Nickell, supposed alien implants appear to be ordinary materials such as a shard of glass, a jagged piece of metal, and a carbon fiber.
Nickell cites Israeli teaching hospital department head Virgil Priscu's opinion that there's "No mystery, no implants", explaining that normal objects picked up during a fall or by walking barefoot often become surrounded by scar tissue.
[2] Alien implants, pictured as small needles inserted into victim's necks and stomachs, first appeared in the 1953 film Invaders from Mars.