When we answer a question about a story, experience we've had or favourite food, it's nice to add extra information that the reader might find helpful. Sometimes your plus will be facts you know about the topic from, say a tv show you saw or a book you read, other times, a plus might be an extra detail that makes your writing more interesting or entertaining.
Here are two examples where students have answered the question and added their own plus.
How did daily practice help Wayne Gretzky become a better hockey player?
Daily practice helped Wayne Gretzky become a better hockey player. I know this because in the text it said that he was able to practice any time he wanted on his backyard rink that his father built. I am better at piano because I have a keyboard at home. If I only practiced at my lessons I would not have much opportunity to get better.
What does your family do on Christmas Eve?
Christmas Eve with my family is really fun. My grandparents come over and we open presents from them and then we eat all kinds of delicious foods. It gets really fun when my mom puts my whiny brother to bed and my Dad, Nonno and I play video games while my Mom and Nana clean the kitchen.
So you can see that in both examples, the child answered the question but took it a step further by adding that extra information. With the Wayne Gretzky response, the child explained how he can relate to the need for daily practice. In the second response, the child added that funny detail about the whiny brother and the fun really getting started.
In a nutshell, a "plus" is any extra, relevant information that the reader might find useful.