When Portia says, "the quality of mercy is not strain'd; it droppeth as the gentle rain from heaven on the place beneath," she means that you cannot force someone to be merciful. It has to be a free act. She says in her previous line "Then must the Jew be merciful" to which Shylock retorts, "On what compulsion must I? Tell me that." His retort is justified; Portia had no business using the word "must". And she admits it with this famous line. "The quality of mercy" here means the same thing as "the nature of mercy"; its quality is its character.

These are words spoken by Portia in Shakespeare's play The Merchant of Venice, Act 4 Scene 1

This quote comes from Shakespeare's play "The Merchant of Venice." It refers to the quality of mercy, which is described as being like gentle rain that falls from the heavens. It suggests that mercy is a gift from above that has a soothing and calming effect on those who receive it.