Most types of assessments are performed repeatedly (annually, for example). The first time you create a new type of assessment, you need to first create an assessment series. (If you are doing a one-off assessment, you still need to create a series first, but you will only use the series one time.) The reason for grouping assessments into series is so that you can track the results over time.
For example, imagine you want to do an annual organizational risk assessment. You want to see whether your risk profile is improving or getting worse from year to year. So before doing the first assessment, you create an Annual Risk assessment series. Then, each time you do a new annual risk assessment, you assign it to the Annual Risk series and you choose the same set of units to assess. You will probably use the same questionnaire template as well, although over time the template may evolve due to changing requirements.
Another important aspect of assessment series is to allow previous survey responses to propagate forward in time. When a new assessment is launched, there is an option to import previous survey data. If enabled, surveys are pre-filled with the answers from last time the same assessment was run, within the same series.
See also: Setting Up Assessments