The main plot of Shakespeare's The Merchant of Venice is the Antonio-Bassanio-Shylock plot, in which the merchant Antonio borrows money from the Jewish moneylender Shylock in order to fund Bassanio's courtship. Along the way, Shylock convinces Antonio to agree to giving him a pound of flesh if he can't pay back the loan. This plot is the main focus of the story, but it also relies on two subplots to progress.
First of all, there is the subplot involving Portia, her suitors, and the three caskets. In this subplot, a series of suitors try to guess the correct casket and win the right to marry Portia, a rich heiress. This subplot is interwoven with the main plot because Portia is the woman Bassanio aims to woo, and so Antonio only borrows the money from Shylock in order to help Bassanio fund his attempt to win the heiress of Belmont's love.
Second, there is a subplot involving Lorenzo, a friend of Bassanio and Antonio, and Jessica, Shylock's daughter. Lorenzo and Jessica elope, enraging Shylock. This second subplot connects to the main plot because, since Shylock is upset with Lorenzo for stealing away his daughter, he's in a particularly foul mood, especially when it comes to Lorenzo's friends, Antonio and Bassanio. Thus, when Antonio fails to pay the loan, Shylock is only too happy to exact his revenge. Thus begins the main climax of the story and the famous trial scene in which Portia comes to Antonio's rescue.
Therefore, we can see that the main plot of the play relies on two interwoven subplots to progress, and the ways in which Shakespeare brings these seemingly diverse stories together is quite masterful.