The Tracker is a device used by S.H.I.E.L.D. to transmit the location of the item or person to which it is attached.
Upon discovering that Ian Quinn had recently purchased an unknown object developed by Cybertek, Coulson's Team planned to board the train transporting it in an undercover mission, in order to follow the package and trap Quinn. The purpose of the mission was to attach a Tracker to the package, and then safely follow it to Quinn's location.
However, Luca Russo informed Cybertek of the team's presence inside the train, where they were attacked and forced to separate. Leo Fitz and Skye managed to follow the package to the compound where Quinn was receiving it. Before Skye entered the compound, Fitz activated the Tracker to let the rest of the team know their current location.
Tracking Lincoln Campbell
During a conversation between Daisy Johnson and Lincoln Campbell, where she attempted to ask him to be the transitioner for Joey Gutierrez, Campbell refused Johnson's request and attempted to walk away from her. Alphonso Mackenzie grabbed Campbell's arm, telling him that he would let Johnson finish her thought. Unknown to Campbell, Mackenzie placed a Tracker on him.
Using a Tracker, S.H.I.E.L.D. was able to locate Campbell when he was on the run from the Advanced Threat Containment Unit. Receiving information about Campbell's whereabouts and his phone number, Johnson contacted him, attempting to convince Campbell to let S.H.I.E.L.D. help him. Campbell quickly figured out about his Tracker and used an electric charge to deactivate it.