It indicates the importance of work (e.g. homework, test, lab, exam, etc.) performed during study process.
That means that quizzes and essays are more important than homework, whereas midterm and final exam count for the same percentage of your grade as all of your homework, quizzes and essays combined.
The total sum of weights should be 100%.
To add weights in iStudiez Pro go to the Planner > Semester > Course > Grades > Weights - add new weights.
If your institution does not use Weights system do not add weights or create one 100% weight, as it may affect your GPA calculation.