This is the 8917th most frequent Polish word.
siergieja
"Siergieja" is the genitive or accusative form of "Siergiej," a Polish version of the name "Sergei."
Here, 'Siergieja' is an accusative case for a proper noun, the Polish variant of the name Sergei, as it's the object of the sentence.
Siergieja znalazłem w parku wczoraj wieczorem.
I found Siergiej in the park yesterday evening.
In this sentence, 'Siergieja' is again the accusative case of the name Siergiej, used after the verb 'przedstawię.'
Przedstawię ci Siergieja, mojego przyjaciela.
I will introduce you to Siergiej, my friend.
Here, 'Siergieja' is used in the accusative case for the name Siergiej, as it follows the verb 'widzieć' which demands a direct object in this form.