This is the 14513th most frequent Russian word.
папу
"Dad" or "Daddy" (in the accusative case).
The word "папу" in this sentence is the accusative form of "папа" (Dad), used because "папа" is the direct object of the verb "вижу" (see).
Я вижу папу в парке.
I see Dad in the park.
Here, "папу" is intended in the accusative case following the context of the verb and intention in expressing the purpose of the action.
Он купил подарок для папу.
He bought a gift for Dad.
The word "папу" is in the accusative case, indicating that Dad is the one being called upon by Mom.