The Time, Elevation, Latitude, and Longitude is a set of variables used to determine when and where the historical death of a person takes place. It is used by Travelers from the future to download their consciousness to that person, moments before the historical death is supposed to happen.
The first mention of T.E.L.L. is when FBI Agent Forbes informs his partner, pre-Traveler FBI Special Agent Grant MacLaren, about a potential cell flagged due to encrypted communication in the deep web. T.E.L.L. is explicitly defined during the last moments in the episode when the group explains the mission to MacLaren just before his body is taken over by Traveler 3468. (S1E01: Travelers)
During the Director's attempts to accomplish a Protocol Alpha mission, the Elevation part of Carrie's T.E.L.L. is revised multiple times to accommodate the skydiving host's descent to the ground, using distance from the plane as the basis. As successive Travelers attempt to download into Carrie, the elevation variable increases. (S2E07: 17 Minutes)