A holo is a device used by the rebels to display Plutarch Heavensbee's holograph, that showed locations of known pods hidden in the Capitol streets. They are able to zoom in on the streets and see which pods are nearby.
A holo can only be activated by a specific commander's voice. Once on, it responds to the other voices from the squadron so someone else can take over if the commander is killed. If anyone in the squadron flicks the switch and say "nightlock" three times in a row, the Holo explodes and blows up everything in a five-yard radius, which serves the purpose of killing them if they get caught and preventing the Holo from falling into enemy hands, and the users getting tortured for information.
As the leader of Squad 451, Boggs was issued a Holo. After he stepped on a land mine and was mortally wounded, he transferred the prime security clearance over to Katniss Everdeen. This bypassed the squad's second in command, Jackson, and effectively made Katniss the new squad leader. Jackson later disputed this, and half the squad backed her while the other half backed Katniss. However she relented when Katniss claimed that she had orders from President Alma Coin to assassinate President Coriolanus Snow and was supported by Cressida, who lied that Plutarch Heavensbee wanted the assassination televised. The squad was pursued by lizard muttations in the Transfer, and Katniss detonated the Holo after the muttations killed Finnick, effectively covering their tracks, in the hope that Snow would not find them.