The Wednesday Wars is about Holling Hoodhood's seventh-grade year; he spends most of his Wednesday afternoons reading and discussing Shakespeare plays with his teacher, Mrs. Baker. The line "the quality of mercy is not strain'd" comes from the first play she has Hollling read, The Merchant of Venice.
At this point in the novel, Holling is still convinced that Mrs. Baker hates him and assumes she is having him read this play because she hopes it will bore him senseless. If that was her goal, she was not successful. Holling really likes the play, and he especially likes the trial in which Shylock is trying to claim his pound of flesh from Antonio in court. In fact, he compares it with his favorite (until now) thing to read, Treasure Island. Just when Holling assumes Antonio is going to have to pay with his life, Portia comes to the rescue and gives a moving and successful speech in which she says the following:
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.
After reading the lines, Holling is moved and says,
Those are words to make you shiver, So, another nefarious Mrs. Baker plot foiled.
The point Portia makes in these lines is first, that mercy cannot be forced ("strain'd" or constrained) and second, that mercy blesses both the recipient and the mercy-giver. Specifically, then, the line "the quality of mercy is not strain'd" means that no one can force another person to be merciful or show mercy. In this play, Shylock does not show mercy and he is sorely punished because of it.
it means that mercy cannot be forced it must be freely given