This is the 730th most frequent Spanish word.
la
"The" (feminine singular definite article).
In this sentence, 'la' is used as a definite article, denoting the feminine singular noun 'casa'.
La casa es grande.
The house is big.
Here, 'la' is used twice: first as a definite article for 'profesora' (feminine singular noun), and then as a definite article for 'escuela' (feminine singular noun).
Ví a la profesora en la escuela.
I saw the teacher at school.
In this sentence, 'la' functions as a direct object pronoun, referring to a specific feminine singular noun previously mentioned or implied.