This is the 8th most frequent Spanish word.
una
"Una" is the feminine singular form of "a" or "an."
Here, 'una' is used as the feminine indefinite article for the noun 'manzana' (apple). It specifies one apple in a general sense.
Ella compró una manzana.
She bought an apple.
In this case, 'una' is serving as the feminine indefinite article for 'sorpresa' (surprise), conveying the meaning of one unspecified surprise.
Es una sorpresa para todos.
It's a surprise for everyone.
Here, 'una' acts as the feminine indefinite article associated with the noun 'razón' (reason), indicating a specific but not previously mentioned reason.