This is the 22nd most frequent Spanish word.
dos
The number "two."
In this sentence, 'dos' is used as a numeral to quantify the noun 'manzanas' (apples).
Voy a comprar dos manzanas en la tienda.
I am going to buy two apples at the store.
Here, 'dos' is again a numeral indicating the amount of time in hours.
Ellos volverán en dos horas.
They will return in two hours.
In this case, 'dos' is part of the phrase 'dos de espadas' referring to a specific card in a deck, used metaphorically to describe the relationship.