I can't find any evidence of 'tag slot' as a specific software term, so it probably is just a 'slot for a tag', i.e. some empty space or placeholder where a tag can be inserted/placed. Presumably, by clicking on that 'slot' (perhaps some kind of little box on the screen?) the user can fill the slot with the tag corresponding to that tag option.

But without more context it's hard to be sure. Is this HTML software or something similar?