The recording-translating device was a multi-function communicator provided to Captain James T. Kirk and the Gorn Captain by the Metrons, for use in their matched combat following the destruction of the Federation outpost on Cestus III in 2267.
The functions of the device were not explained in detail to Kirk, who used it as a log recorder at first. The local two-way communications and universal translator feature was discovered early-on by his Gorn opponent, who used it to eavesdrop on Kirk's musings. The device facilitated Kirk's understanding of the Gorn's mission, to defend the Gorn Hegemony from a perceived Federation invasion.
When struck against a lump of coal, the device also served as a useful firestarter, igniting the fuse that fired Kirk's primitive cannon at the advancing Gorn, disabling him. (TOS: "Arena")