Antonio wanted to help his bestfriend bassanio to woo portia, a rich heiress. as he didn't have any money at hand, he borrowed three thousand ducats from shylock, a jewish moneylender. shylock and antonio were sworn enemies and shylock wanted to take revenge. antonio signed a bond stating that if he did not return the money on a stipulated date, shylock would cut off a pound of his flesh from whatever part he desired.
Antonio was a generous man, always willing to help his friends. His dependence on bassanio, his only real affection and emotional prop, leads him to extreame self-sacrifice. He was ready to do anything for Bassanio.
So, for indepted bassanio, Antonio took loan from the jew moneylender, Shylock, who was his greatest foe. bassanio needed money to set himself as'gentle man', and so asks for help to Antonio. Antonio did'nt had ready cash, so he asks Bassanio to locate a moneylender who will help them. Ironically, Bassanio finds shylock who was been waiting for Antonio for revenge.
Antonio asks for a loan of 3000 ducats. Then he had to promise his life to Shylock he fails to return the money within 3months. This was the Fatal bond.

hope this helps...