How does it actually work? Just one example below
Lets say you want to find a end of module final mark. You have 2 total exams/assignments for that module. One worth 40% weighting and you get 75/100 marks, the other worth 60% weighting and you get 62/100. So you can type in 40 for the weight and 75 for the grade on first row, also 60 and 62 for the next row. Press Calculate and you find you got an overall average mark of 67.2 and a grade of 2:1 for that module.
Grades that are in decimals are always rounded up (check with your university how they address this), e.g 69.9 is rounded up to 70.