This is the 200th most frequent Finnish word.
läpi
"Through."
Here, 'läpi' is used as a preposition meaning 'through', indicating movement across a space.
Mies käveli metsän läpi.
The man walked through the forest.
In this sentence, 'läpi' again functions as a preposition, describing vision passing through an object.
Hän katsoi ikkunan läpi.
He/she looked through the window.
'Läpi' here conveys the meaning of 'throughout', indicating the entirety of a time period.