This is the 291st most frequent Swedish word.
vissa
"Vissa" in Swedish means "certain" or "some."
Here, 'vissa' is used as an indefinite pronoun meaning 'some' to indicate an unspecified subset of people.
Vissa människor gillar att läsa böcker.
Some people like to read books.
'Vissa' functions here as in the first example, denoting 'some' to signify a subset of days in an undefined manner.
Vissa dagar känns allt lättare.
Some days, everything feels easier.
In this sentence, 'vissa' means 'certain' and is used to specify particular documents from a known or implied set.