This is the 16614th most frequent Russian word.
селедкой
"Селедкой" (instrumental case of "селедка") means "herring" (a type of fish).
In this sentence, 'селедку' is used to refer to the food item, herring, in the accusative case as it is the direct object of the verb 'ели'.
На обед мы ели селедку с картошкой.
For lunch, we ate herring with potatoes.
Here, 'селедку' is again used to denote herring, being placed on the table, illustrating its usage as an object in a culinary context.
Она накрыла стол и поставила селедку.
She set the table and placed the herring.
In this case, 'селедкой' appears in the instrumental case, indicating the reason for the restaurant's reputation.