This is the 13575th most frequent Polish word.
piszącego
"Piszącego" means "writing" or "of the one who is writing" in English.
The word 'piszącego' is used as an adjective to describe 'chłopca' (the boy) who is performing the action of writing.
Widziałem chłopca piszącego list do swojego przyjaciela.
I saw a boy writing a letter to his friend.
Here, 'piszącym' is an adjectival participle describing 'człowiekiem' (the person) who is writing something.
Rozmawiałem z człowiekiem piszącym książkę o historii miasta.
I spoke with a person writing a book about the city's history.
In this instance, 'piszącego' characterizes 'kogoś' (someone) as performing the act of writing.