This is the 12136th most frequent Russian word.
икру
The Russian word "икру" means "caviar" or "fish roe" in English.
In this case, 'с икрой' makes use of the instrumental case, meaning 'with caviar,' to describe the sandwiches.
Мы ели бутерброды с икрой на завтрак.
We ate sandwiches with caviar for breakfast.
The word 'икру' is used in the accusative case as the direct object of the verb 'купил' (bought) referring to caviar.
Я купил икру для праздничного ужина.
I bought caviar for the festive dinner.
Here, 'икру' denotes fish roe, used in the accusative case as the object of the verb 'мечет' (spawns).