Well, if you are working on an image the intensity values for the pixels will have a certain frequency.
10 pixels will have the intensity value of 0, 25 pixels the value of 1, etc.
The cumulative frequency is the sum of the previous frequencies including the current intensity
So you will have a table like this:
Intensity | Frequency | Cumulative Frequency | Calculation |
0 | 10 | 10 | 0 + 10 |
1 | 25 | 35 | 10 + 25 |
2 | 30 | 65 | 35 + 30 |
3 | 35 | 100 | 65 + 35 |
4 | 30 | 130 | 100 + 30 |
etc. | | | |