A cohort is a group of subjects or people who share at least one common feature or characteristic, over a given duration of time. A cohort study is an observational study, where we try to correlate a condition with potential causal factors. This can be both prospective (looking forward) and retrospective (looking backwards).
In this case, the cohort study was carried out progressively, which means the cohort was observed over a period of time and these observations were used to test the hypothesis. The other way could have been to identify the affected cases and carry out the study retrospectively. An alternate option would have been to conduct a randomized controlled trial (RCT), in which each subject is randomly assigned to a control group (no treatment or change) or target group (treated or exposed to something, etc.). Using RCT in current scenario would have been a challenge, since we cannot expose any subject to causal factors, i.e., arsenic containing water (that would be against human and research ethics). To overcome those limitations, a prospective cohort study was the best option, even though it took a long time to generate results.
Hope this helps.