Antonio helps Bassanio by borrowing money from Shylock, the Jewish money-lender, to loan to Bassanio so that he (Bassanio) can court Portia, a wealthy heiress. Antonio's money is tied up in his ships and trading, so he doesn't have the cash on hand to loan to Bassanio himself. The trouble occurs when Shylock wants a pound of Antonio's flesh, rather than the money paid back and the interest he would have made on the sum.
Antonios finances are all at sea in ships. He has no solid money only cargo. However he has a good credit rating and this allows him to be a guarantor for Bassanio.