In this scene, a supposed argument has ensued between Portia and Bassanio about the fact that he had given away a precious gift that she had presented to him just before their marriage. The gift was a ring that she had given him as an act of love. She had made Bassanio swear that the ring would never leave his finger as an indication of his love and loyalty to her.
This house, these servants and this same myself
Are yours, my lord: I give them with this ring;
Which when you part from, lose, or give away,
Let it presage the ruin of your love
And be my vantage to exclaim on you.
Bassanio had felt compelled to present the ring to the young lawyer who had defended Antonio against Shylock's malevolent desire to take revenge. Bassanio gave the ring out of gratitude since the lawyer (who was actually Portia in disguise) would not accept any sum of money but was prepared to accept the ring as reward.
You press me far, and therefore I will yield.
Give me your gloves, I'll wear them for your sake;
And, for your love, I'll take this ring from you:
Do not draw back your hand; I'll take no more;
And you in love shall not deny me this.
This ring, good sir, alas, it is a trifle!
I will not shame myself to give you this.
I will have nothing else but only this;
And now methinks I have a mind to it.
Portia continues the charade that she is extremely displeased that Bassanio could so easily break a solemn promise and give away her precious gift. He is obviously distraught about the matter and promises her that he will never break an oath to her ever again. Antonio intervenes at this stage and says the following:
I once did lend my body for his wealth;
Which, but for him that had your husband's ring,
Had quite miscarried: I dare be bound again,
My soul upon the forfeit, that your lord
Will never more break faith advisedly.
He states that he had once lent his body to secure Bassanio a benefit but that that arrangement had gone awry, except for the lawyer (in effect Portia) that had acquired the ring in question.
Antonio promises that he will be held to a bond again and would forfeit his soul on the guarantee that Bassanio will never again foolishly break a promise. Portia then accepts Antonio as a surety and hands him the ring that Bassanio assumed he was giving to a stranger.
When Bassanio realises that she had just handed over the very ring he had 'given away' Portia continues with her little game and tells him that she had slept with the doctor who had then given her the ring. Nerissa, who had been playing along and had accused Gratiano of doing the same that Bassanio did, also 'confesses' that she had done the same with the lawyer's clerk (herself in disguise).
Both Bassanio and Gratiano are shocked by the news and Gratiano wants to know if both he and Bassanio had been cuckolded even before they deserved it. Portia only then comes out with the truth and shows the two men the letter from Bellario in which it is clearly stated that 'Portia was the doctor Nerissa there her clerk.' Antonio declares that he is speechless and the two new bridegrooms express their relief after making sure that the two women were truly who they claimed to have been at Antonio's trial.
And so, all's well that ends well.