“à” is a very common preposition. It has three forms: à, au, and aux. We use “à” when the noun that follows it is singular feminine preceded with the definite article “la”.
J’ai trouvé un livre très intéressant à la bibliothèque. I found a very interesting book at the library.
J’ai un rendez-vous important à la banque. I have an important appointment at the bank.
We use “au” when the noun that follows it is singular masculine.
Nous avons passé toute la journée au musée. We spent the whole day at the museum.