In this status-conscious society, money is everything. Bassanio may be an aristocrat, he may be head over heels in love with Portia, but he knows that's not enough. If he's to keep her in a style to which she's become accustomed, he's going to need to get his hands on some ready cash. In Venice, wooing a lady—especially one of such wealth and social standing as Portia—requires money, and lots of it.
Unfortunately, Bassanio doesn't have much money of his own, so he goes to borrow some from his old friend Antonio. Antonio's a bit of a soft touch and has lent money to Bassanio in the past. Although all his money's currently tied up in his investments, Antonio agrees to help out Bassanio once again. Only he won't be able to lend him money out of his own pocket this time; he'll have to borrow it from Shylock, the moneylender, at very disadvantageous terms.