This is the 12395th most frequent Russian word.
картошке
"Картошке" means "to potato" (dative case of "картошка," meaning "potato").
Here, 'картошке' is in the prepositional case to indicate the location where the butter is.
Я люблю масло на картошке.
I love butter on potatoes.
In this sentence, 'картошке' is in the instrumental case used after the preposition 'с' to denote accompaniment or proximity.
Мы сидели рядом с картошкой.
We were sitting next to the potatoes.
Here, 'картошке' is again in the prepositional case, implying the topic of discussion in relation to 'по', which denotes a subject matter.