This is the 3rd most frequent Italian word.
la
"The" (feminine singular definite article).
In this sentence, 'la' is a definite article referring to a specific feminine singular noun, 'ragazza', which means 'girl'.
La ragazza cammina nel parco.
The girl walks in the park.
Here, 'la' is again a definite article used to specify the feminine singular noun 'mappa', which translates to 'map'.
Ho incluso la mappa nel libro.
I included the map in the book.
In this case, 'la' acts as a direct object pronoun, referring to a feminine singular person or object that is being seen.