This is the 886th most frequent Polish word.
rok
"Rok" in Polish means "year" in English.
'Rok' here is in the nominative case as the subject of the sentence, which identifies a specific year.
Rok 1999 był dla mnie bardzo ważny.
The year 1999 was very important to me.
In this sentence, 'roku' is used in the locative case to indicate a point in time, 'during last year'.
W zeszłym roku zrobiłem prawo jazdy.
Last year I got my driving license.
Here 'roku' is in the genitive case to express the idea 'of each year'.