Students are often by confused by weighted grades, but it’s not that complicated.
All grades are “weighted” in one way or another. For example, let’s say you have ten assignments in a course for which you can earn a grade. If the grades for these assignments are weighed equally each would be worth the same number of points or the same percentage of the total course grade (e.g., 10 points each or 10% each, etc.).
But more typically the various assignments in a course are not weighed equally. This fact usually reflects the instructor’s priorities in the course. For example, in almost all of my courses the term paper assignment is weighed the most heavily because I think writing a good philosophy paper is the best indicator of whether or not the student is “getting” the topic of a course. By way of contrast, weekly reading quizzes are weighed rather lightly because I don’t think they’re anywhere near as important in indicating the student’s overall performance in a course.
Now what this means of course is that getting a good grade on a heavily-weighed assignment is going to help your overall course grade a lot, but getting a bad grade on a it will hurt your overall grade a lot. The opposite would be true for a lightly-weighed assignments: tanking a reading quiz worth 1% of your grade isn’t going to hurt you much, and acing it isn’t going to help you much either. (Still, the point is to do well on all your assignments—right?)
Finally, you might ask why I use percentages instead of just allocating different numbers of points for each assignment. The answer on one level is simple: I just prefer percentages to points. The highest number of points a student can get on any assignment is 100, but since each assignment is weighed differently so not all 100s are of equal value. The highest number of points a student can get in a course is 100, and this final course grade is the sum of all these differently-weighed 100-point assignments. Furthermore, the 100-point scale is widely familiar to students, it’s easy to correlate to the four-point A-F scale, etc.