This tab appears only if to set value in tab Settings -> Languages. On the screen above value is: en_US,es_ES.
In the tab "Vocabulary" it is possible to translate words which you applied in filters(titles) and also taxonomies terms.
To add a word into Vocabulary use button "Create" and write there exact word or sentence you want to translate. To find any word quickly use text input on the table top.
An example translation of the word "Size" into Spanish: