This is the 272nd most frequent Polish word.
kolejnych
The Polish word "kolejnych" means "subsequent" or "next" in English, depending on context.
Here, 'kolejne' (inflected as 'kolejnych' in its genitive form when used) refers to 'next' or 'subsequent', modifying 'wakacje' (holidays) to discuss future endeavors.
Spędzimy kolejne wakacje nad morzem.
We will spend the next holidays by the sea.
In this sentence, 'kolejnych' is the genitive plural form of 'kolejne', used here to describe the subsequent 'dni' (days) after a specific period.
W trakcie kolejnych dni pracowaliśmy nad projektem.
During the following days, we worked on the project.
'Kolejne' here, when inflected as 'kolejnych' for the accusative plural, refers to 'some more' or 'additional', modifying 'książki' (books), discussing adding to a group that already exists.