Explain Portia's famous speech ("The quality of mercy is not strained...") and what it says of her character.

It's one of those Shakespeare speeches, I think, like "To be or not to be", where everyone knows the first line or two lines, and then very little about what comes next. Portia is talking about mercy, which, she argues

...blesseth him that gives, and him that takes:
'Tis mightiest in the mightiest; it becomes
The throned monarch better than his crown

Being merciful, she argues, blesses the person being merciful, and the person who receives the mercy. It is, therefore at its strongest, shown by people who are the most powerful. It becomes a monarch (i.e. is more kingly) better than his crown does.

His sceptre shows the force of temporal power,
The attribute to awe and majesty,
Wherein doth sit the dread and fear of kings;
But mercy is above this sceptred sway,

The king's sceptre shows the force of his power on earth ("temporal power"), and is part of his awe and his majesty, which is why people dread and are scared of kings. However, mercy is above the influence and power of the sceptre ("sceptred sway")...

It is an attribute to God himself;
And earthly power doth then show likest God's
When mercy seasons justice.

Mercy is an attribute of God himself, the king of kings. And earthly power becomes most godly when it is merciful: when decisions of justice are "seasoned" (made more palatable) with mercy.

Hope it helps!