The Jamarat are three stone pillars which are pelted as a compulsory ritual of Hajj in emulation of the Prophet Ebrahim (upon him be peace). They represent the three locations where Ebrahim (upon him be peace) pelted the Shaitan (Satan) with stones when he tried to dissuade him from sacrificing his son Ismail (upon him be peace).
Allah (Glorified and Exalted is He) was pleased with the sincerity of Ebrahim (upon him be peace) and sent a white, big-eyed sheep with horns in replacement of Ismail (upon him be peace) which Ebrahim (upon him be peace) sacrificed instead. This is the basis for the offering of sacrifice animals by the Hujjaj and by all other Muslims at the time of Eid-ul-Adha.