This is the 530th most frequent Finnish word.
pieni
Small.
The word 'pieni' is used here as an adjective to describe the noun 'talo' (house), referring to its small size.
Tämä on pieni talo.
This is a small house.
Here, 'pieni' describes the noun 'virheen' (mistake), indicating that the mistake is minor or insignificant.
Hän teki pienen virheen.
He made a small mistake.
In this case, 'pienet' is the plural form of the adjective 'pieni', agreeing in number with the noun 'asiat' (things).