This is the 4156th most frequent Russian word.
сто
"Sto" in Russian means "hundred."
In this sentence, 'сто' is used as a numeral indicating the quantity 'hundred'.
На дороге стоит знак, который говорит: ограничение скорости сто километров в час.
On the road, there is a sign that says: speed limit of hundred kilometers per hour.
Here, 'сто' again represents the numeral 'hundred', used to quantify currency, rubles.
Он готов заплатить сто рублей за эту книгу.
He is ready to pay hundred rubles for this book.
Although 'сто' does not explicitly appear, this sentence helps understand 'стоять', the verb form related to standing, which 'сто' is derived from.