The short answer is Organic Traffic is an estimated monthly figure based on the past 30 days. Read on to understand the nuances in the precision of this figure.
We calculate the organic traffic of each keyword at the respective ranking position of the target website/URL by multiplying the estimated search volume of the keyword and the unique CTR of the target website/URL's position in a SERP.
The key thing to note is that not all target URL/domain's position in a SERP is updated at the same time. We update the target URL/domain's position in a SERP, once in a few days to once in 1-2 months depending on the popularity of the respective keyword.
So in other words, a more popular keyword gets updated more frequently than a relatively less popular keyword. For example, a popular keyword that is updated once in a few days will get timely monthly data. However, a less popular keyword that is updated once in 2 months will get data at the last time of update, which may be a little longer than the previous 30 days.
Thus, Organic Traffic is really our best estimated monthly figure based on the past 30 days, bearing such nuances in the frequency of update of certain less popular keywords.