Within ArcGIS Online you can publish a CSV file as a Table, which enables you to apply certain filters or to specify the data type of each column. This can be useful if you wish to display categoric values left aligned in a Report Builder Table widget for example. Report Builder will guess the data type of values added through a CSV file but it might not always get it right e.g. US Zip Codes would be treated as numbers and hence displayed right aligned.
Tables have a further advantage in that if your CSV is very large then it can become slow. The whole CSV file has to be downloaded in order to do anything with it. However, when the CSV is converted into a Table it is queried by ArcGIS Online and only pulls back the data that is being used by particular widgets.
When adding an ArcGIS Online Table to your report data, it is important that the Table has a column named the same as the Feature ID field from your feature service. This column needs to contain matching feature codes.