This is the 10th most frequent Polish word.
roku
"Roku" in Polish means "of the year."
Here, 'roku' is used in the context of time, referring to 'roku' as the genitive singular form of 'rok' (year), used after 'w' (in).
Pierwszy dzień w roku jest zawsze w styczniu.
The first day of the year is always in January.
'Roku' here refers to 'this year' and is influenced by 'w tym' (in this), again the genitive singular form.
Byliśmy na wakacjach dwa razy w tym roku.
We went on vacation twice this year.
In this sentence, 'roku' is a temporal expression signifying 'last year', also in the genitive singular form after 'w zeszłym' (in the past).