A planbook is a container for all the classes/subjects you're teaching for one term. For some teachers a term might be a semester, for other teachers, a term might be an entire year!
If you're a middle or high school teacher on a semester system, you might make two planbooks this year: one that you call something like "Math -- Fall 2017" that has all of your fall classes in it, and one called "Math -- Spring 2018" that has all your spring classes in it. If you are a middle or high school teacher who keeps the same classes all year, you will just want to make one planbook, called something like, “Math 2017-2018” that will have all your classes for the entire year.
An elementary teacher on the other hand will probably have just one planbook this year, maybe called something like "3rd Grade 2017-2018". That planbook would have all of his or her classes/subjects for the entire year in it.
Note: One planbook per term is a good general rule, but some people might end up making two or more per term (for example, someone that teaches after-school classes might want to keep their main planbook and their after-school planbooks separate). If you have any questions on how you should set up your planbooks, reach out to email@example.com.