You start with the first value, and the add all following values one by one.
Assume, for example, that I collect some apples from a tree. I collect 3 apples on monday, 4 on tuesday, 2 on wednesday, 6 on thursday, 1 on friday, 0 on saturday and 1 on sunday.
You can think of the cumulative frequency as the number of apples collected so far. So, the cumulative frequency on monday is the number of apples collected from the beginning of the week until monday, i.e. monday alone. So, it is 3.
The cumulative frequency on tuesday is the number of apples collected from the beginning of the week until tuesday, i.e. those collected on monday plus those collected on tuesday. So, it is 7.
And so on. With my dataset, the cumulative frequency is
