This is the 335th most frequent Finnish word.
noin
Approximately; about; around.
In this case, 'noin' expresses a completion or satisfaction, akin to saying 'there' or 'all done' in English.
Noin, se on valmis!
There, it's done!
In this sentence, 'noin' is used as an adverb to mean 'approximately' and is used to estimate the distance walked without specifying an exact measurement.
Menimme noin kilometrin kävellen.
We walked approximately a kilometer.
Here, 'noin' is used to describe the manner of how something happened or occurred, and it carries an implication of demonstrating or indicating 'in that way'.