This is the 611th most frequent Polish word.
wrocławiu
"Wrocławiu" is the locative case of "Wrocław," meaning "in Wrocław," a city in Poland.
Here, 'wrocławiu' also appears in the locative case, indicating the location where the event (meeting) happened, combined with the preposition 'w'.
Spotkałem ją w Wrocławiu podczas wakacji.
I met her in Wrocław during the holidays.
The word 'wrocławiu' in this instance is the locative case form of the city's name 'Wrocław,' used to indicate location when preceded by a preposition.
Mieszkam we Wrocławiu od pięciu lat.
I have been living in Wrocław for five years.
In this case, 'Wrocławiu' is used in the vocative case, showing direct address to Wrocław as if the speaker is speaking to the city directly.