This is the 4933rd most frequent Polish word.
plastiku
"Plastiku" means "plastic" (material) in English.
The word "plastiku" is used as a noun in the genitive case to indicate the material something is made from.
Butelka jest wykonana z plastiku.
The bottle is made of plastic.
Here, "plastiku" is again in the genitive case, which is required after the verb "wyrzucać" in this context, denoting the material being disposed of.
Nie wyrzucaj plastiku do zwykłego kosza.
Do not throw plastic into the regular trash bin.
The genitive "plastiku" is used to describe what the prohibition applies to (usage of a material).