This is the 3900th most frequent Polish word.
ryb
"Ryb" is the genitive plural form of "ryba," meaning "fish" in Polish.
The word 'ryb' here is in the genitive plural case, used after the adjective 'świeżych' indicating a partitive or kind of description.
Lubię smak świeżych ryb.
I like the taste of fresh fish.
The word 'ryb' is in the genitive plural case specifying a quantity after 'dużo' (which requires genitive).
W rzece było dużo ryb.
There were many fish in the river.
The word 'ryb' is in the genitive plural case following the quantified number, here 'piętnaście' (which governs the genitive plural).