This is the 16th most frequent Greek word.
ένα
"Ένα" means "one" in English.
In this sentence, "ένα" is used as an indefinite article preceding a singular, masculine or neuter noun "βιβλίο".
Αγόρασα ένα βιβλίο.
I bought a book.
Here, "ένα" works as the numeral 'one,' specifying the quantity of "ευρώ" found.
Βρήκα ένα ευρώ κάτω από το τραπέζι.
I found one euro under the table.
In this example, "ένα" functions as an indefinite article applied to the noun "παιχνίδι" to denote an unspecified single item.