This is the 8487th most frequent Polish word.
porzeczki
"Porzeczki" means "currants" in English.
Here, 'porzeczki' refers to the fruit{'s name' in its plural form, used as the subject of the sentence.
Porzeczki są moim ulubionym owocem.
Currants are my favorite fruit.
In this case, 'porzeczkami' is the instrumental plural form, denoting the ingredient used in the cake.
Mama upiekła pyszny tort z porzeczkami.
Mom baked a delicious cake with currants.
Here, 'porzeczek', the genitive plural form is used in reference to the bushes of currants.