This is the 122nd most frequent Finnish word.
kerran
"Once."
Here, 'kerran' means 'once', describing a single occurrence of the action.
Menimme elokuviin kerran viime kuussa.
We went to the movies once last month.
In this sentence, 'kerran' is used in an illustrative context to denote a hypothetical single occurrence.
Kerro koko tarina alusta asti niin kuin olisin kuullut sen vain kerran.
Tell the whole story from the beginning as if I had only heard it once.
Here, 'kerran' sets the scene for a story or memory, indicating a specific past occurrence.