Mood is the feeling the audience receives from what the author has written. Sometimes it encompasses the atmosphere of a book.
As an 11th grader, I believe you have likely read by now both Romeo and Juliet and To Kill a Mockingbird.
In Romeo and Juliet, the end of the beginning scene paints a picture of Romeo and Benvolio talking with Benvolio seeking an answer to what is bothering Romeo. Throughout this scene, we feel depressed for him, we wonder if he will be able to overcome confusion, and we relate to rejection. This mood is melancholy or depressing.
In the courtroom scene in To Kill a Mockingbird, Atticus Finch begins to take off things that are making him too hot that day. Now, it could have been just a hot day, or this could have been the most important case he ever tried. I believe the latter. Atticus, the coolest lawyer character in all literary history, sweat his way through this trial. He took off his coat, unbuttoned the top button to his shirt and even loosened his tie. This makes us evaluate the situation unconsciously and realize the TENSE moment he is dealing with.
These two pieces are ones I would think any Junior in the USA could relate to and had at least gone over in class. I hope the moods of sadness and intensity are ones you can relate to in each of these pieces.