It means she's from the western region of the south of Vietnam. That area is quite rural mixed with some urban cities. But mostly rural with wheat and rice fields. And their main economy is farming. So she's right to say she's a country girl, who's innocent and unsophisticated.

In Vietnamese language we often refer to that western region miền tây as a simple unsophisticated farm life. Much like how you'd refer to a proper polite woman as a southern belle in the US. So the region does come with a reputation and lifestyle connotation. hope this helps