Firstly, the title is significant because it is a reference to the protagonist and hero of the play, Antonio. The title tells us who the play is about and also provides the setting. Since Antonio does live in Venice and is a merchant, and most of the events in the play revolve around him, we can confidently assume that the title refers to him. Furthermore, most of the action in the drama plays out in the city of Venice, so the title is also apt in this regard.
The play opens in a street in Venice and Antonio is the first character to speak. He is sad and does not know why, but wishes to find the reason for his melancholy:
SCENE I. Venice. A street.Enter ANTONIO, SALARINO, and SALANIO
ANTONIOIn sooth, I know not why I am so sad:
It wearies me; you say it wearies you;
But how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born,
I am to learn;
And such a want-wit sadness makes of me,
That I have much ado to know myself.
We discover that he is a merchant when Salarino opines that Antonio's sadness is the result of his concern about his ships at sea which carry his precious cargo.
SALARINOYour mind is tossing on the ocean;
There, where your argosies with portly sail,
Like signiors and rich burghers on the flood, ...
Antonio quickly assures him, however, that his depressed mood is not caused by concern for his goods:
ANTONIOBelieve me, no: I thank my fortune for it,
My ventures are not in one bottom trusted,
Nor to one place; nor is my whole estate
Upon the fortune of this present year:
Therefore my merchandise makes me not sad.
He assures Salarino that he had not placed all his fortune in only one vessel or that his ships were travelling to only one destination, nor that his entire fortune was dependent on the year's ventures alone, but that he, like a good businessman, had spread his risk. This, therefore, is not the reason for his sadness.