This is the 1st most frequent Polish word.
w
"In" or "at."
The word 'w' here is a preposition used to indicate location within a place.
Mieszkam w Polsce.
I live in Poland.
In this sentence, 'w' indicates being inside an object or container.
Książka jest w torbie.
The book is in the bag.
Here, 'w' functions as a preposition to indicate time, specifically the day of the week.