This is the 71st most frequent German word.
auf
"Auf" means "on," "upon," or "onto" in English, depending on context.
The word 'auf' (on/to) here is used with the accusative case indicating a destination.
Ich gehe auf den Markt.
I am going to the market.
The word 'auf' (on) here is used with the dative case indicating a location.
Das Buch liegt auf dem Tisch.
The book is lying on the table.
The word 'auf' (on/about) here is part of the verb 'sich freuen auf,' which means 'to look forward to' something and requires the accusative case.