To create a query in the tag selector means to select all the tags matching the current filter.
In essence, the Add Query button is a “select all” button, adding all the tags currently shown in the tag selection set to the selection list. To avoid adding too many tags, you should first filter the list. To filter the tag selection list is creating a query.
Rather than populating the Selected Tags list with the individual tags, a text represenation of the query will be shown:

You can view the individual tags that match the query by using the Preview button, as described in the next topic.