A subdermal implant was a device that was implanted under the skin. They can have various purposes.
During the Occupation of Bajor, members of the Bajoran Resistance had subdermal implants composed of tritonium isotopes. In the event of capture, the implants could be activated to leave a trail that could be followed. In 2372, Major Kira Nerys used such a trail to find the survivors of the Ravinok. (DS9: "Indiscretion")