After opening a class from the Classes dashboard, you'll be taken to the class details page where all of your grades are found. Grades are used to determine your class percentage and typically include items such as homework, exams, worksheets, etc.
All of the grades combined produce the overall class score and percentage which is displayed at the top of the class dashboard. Using grades, you're able to insert all of your current grades from your School SIS (such as Powerschool or Schoology) and determine what you need to score in school to see if it meets your requirements.
For example, if you create a fake grade for a test you'll be able to try different score combinations to see what you need to score to maintain your A.