The relief depicts the plundering of the Temple of Jerusalem in 70 CE. It depicts Roman legionnaires carrying the seven-branched candle holder (menorah) from the Temple.
Plundering of the Temple in Jerusalem by the Romans in 70 CE, relief on the Arch of Titus in Rome, colour photograph, 2005, photographer: Anthony M.; source: Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Arch_of_titus_relief_2.jpg.