The technical is an improvised fighting vehicle. That's just a fancy term for "pickup truck with a machinegun mounted on it," or, as I like to call it, a Texan Parade Float. All the technicals on Rook Island belong to Vaas' pirates. They offer plenty of firepower, but no real armor protection.
As mentioned in the description, the only way to get one is to either find a stranded one, or to get it from the pirates. Since there is no windshield, doors or roof, you are heavily exposed to gunfire.
