Portia wants mercy from Shylock in her famous speech of mercy. She talks about the virtue of mercy in a 22 lines long quote. The most important part is the first four lines:

The quality of mercy is not strain’d,
It droppeth as the gentle rain from heaven
Upon the place beneath: it is twice blest;
It blesseth him that gives and him that takes:

Here Portia, in disguise as a young lawyer Balthazar, earnestly requests Shylock to show his mercy for Antonio and forgive him, as mercy is good for both who shows it and who receives it. Mercy is indeed a heavenly quality of a man, as she opines. According to her, it is the strongest and the most powerful people in society who show mercy for others.