"The tasty big sandwiches"
Translation:Smaczne duże kanapki
It's the "niemęskoosobowy" gender (which I think that in this course is usually simply called feminine plural), and as its Polish name says very implicitly, it's used for all plurals apart from groups of male people, or mixed groups of people. This 'feminine plural' has forms identical to singular neuter.
Therefore you will have: wysokie drzewo (a tall tree, neuter singular), wysokie drzewa (tall trees, plural, the gender of the singular word is irrelevant), wysokie policjantki (tall police officers, all female), but wysocy policjanci (tall police officers, either all male or a group of both male and female).