Wraith trackers, also known as a subspace tracking device, are implants used by the Wraith.
These small devices were implanted surgically into the body where they emitted a subspace signal which was capable of being detected by the Wraith. This allowed them to detect the target through the resulting frequency being emitted from the target. They were typically placed close to the spinal column in order to prevent the individual from cutting them out themselves.
Another one was placed in Ronon when he was recaptured by the Wraith and turned back into a Runner and set loose on his homeworld on Sateda. McKay was able to use the device and Atlants' sensors to determine where he was and Sheppard, McKay, Teyla and Dr. Beckett came to his rescue. After Ronon, Sheppard, Teyla and Beckett killed all of the enemy Wraith and escaped in a Puddle Jumper, Beckett removed it from Ronon once again. (SGA: "Sateda")
Todd has shown an affinity for the device. He had a tracker implanted in his body and used it to summon his Hive ship when he was taken captive by the Atlantis expedition at Atlantis. (SGA: "The Seer")
Dr. Keller was kidnapped by another Runner named Kiryk, who wanted her to treat a seriously ill little girl, and offered to remove his tracker from him. However, she was unable to remove it as it was attached to both his spinal column and brain stem. Keller instead shorted it out with a defibrillator, rendering it useless. (SGA: "Tracker")
When the team was placed on trial by the Coalition of Planets, Sheppard asked Richard Woolsey if he had a tracker on him so Atlantis could find them. Woolsey tells them he doesn't as that's Wraith technology before admitting he called Todd for one but didn't get an answer. (SGA: "Inquisition")
When Todd was placed into stasis on Atlantis, Doctors Carson Beckett and Rodney McKay checked him over for trackers. They removed one and the components to make two more. Unknown to them, Todd had more inside of him in a dormant or seedling stage and grew a new tracker while in stasis. This one was described as being crude and more like an emergency beacon. When Todd passed through the Stargate to Manaria, the tracker was activated and signaled Todd's hive ship to come pick him up. Richard Woolsey was displeased at how Todd was able to signal his ship and that when they did come, Sheppard let him go. (SGA: "Legacy: Homecoming")