Let's look at an example of how to work with weightings in the gradebook. Suppose we have all the gradebook items placed in their corresponding categories, and that the total for each of these categories is 100. 

In many cases, the course total corresponds to a grade calculated from these two, meaning each category contributes half of the final grade (if there were 3 categories, each would contribute 33.33%). This is the default setting.

If you want the weights or weightings to be different for each category (for example, you want topic 1 to count for 10%, topic 2 for 20%, etc.), edit the settings for each category so you can assign different weights to the different categories.

Therefore, click Edit on the category (to edit the weight of the items within it) or on the course total (to change the weight of each category) and change that simple weighted mean to another calculation type: weighted mean. This mean will allow you to assign different weights to the different categories/items it contains. If you change the course total to weighted mean, you can weight categories. If you change it in a category, you can weight the items within it, assigning different weights to them.

Then change the weights and save. If you now return to the grader view, you will see that the values of the activities that make up the categories are used to perform the necessary calculations for subtotals and totals according to your configuration.

You will notice that when you edit the scores, the total calculated from them is also updated. Manually edited cells will appear with a colored background to indicate this situation.