This is the 16696th most frequent Russian word.
скамейки
Benches.
In this sentence, "скамейках" is the plural prepositional case of the noun "скамейка", referring to benches as the location of the action.
Дети весело играли на скамейках в парке.
The children were playing joyfully on the benches in the park.
Here, "скамейки" is the plural accusative case of "скамейка", indicating the direct object of the sentence, the benches that were bought.
Мы купили новые скамейки для школьного двора.
We bought new benches for the school yard.
The word "скамейки" is in the nominative plural case, describing the subject, the benches being discussed.