In this speech, Portia pleads with Shylock to be merciful towards Antonio. Justice dictates that since Antonio did not keep his part of the bargain and pay back his loan to Shylock in the allotted time, he should have to give up a pound of flesh, which Shylock wants to cut from his heart. Portia, disguised as a male lawyer, advocates for mercy towards Antonio.
Porita says that the quality of mercy is not strained. By this she means that mercy is not overworked, for many people are cruel, not merciful. She compares mercy to a soft rain that falls from heavens; like rain, mercy falls on us whether we deserve it or not.
Portia also argues that mercy is most moving when the mighty display it and notes that a person with power who shows mercy is acting in the image of God:
And earthly power doth then show likest God's
When mercy seasons justice.
Portia notes, too, that none of us would be saved from paying for our crimes were there no mercy.