The Terms Filter (Aggregator) is used to aggregate terms from strings (text) e.g. Text, Checkbox. It uses text as a base (e.g. Titles) which means that it does not aggregate similar text but only exact matches e.g. List Item "Joe Berrymore" may be present multiple times (e.g. Joe Berrymore, Jo Berrymore, Joe Berymore). We strongly recommend using the refId (with the Terms Relation Filter) as a source when building Filters for Tagbox values, to avoid confusing or wrong results. 

For a multi-value Tagbox (or multi-value Relationship, multi-value Fieldset), it is used in combination with the NestedAggregator.