This is the 104th most frequent Finnish word.
heidän
"Their" (possessive pronoun).
'Heidän' is used as a possessive pronoun indicating ownership of the house.
Heidän talonsa on suuri ja moderni.
Their house is large and modern.
'Heidän' is used to indicate the ones performing the action of singing.
Kuulin heidän laulavan juhlista.
I heard them singing at the party.
'Heidän' is used as a possessive pronoun relating to 'mielipiteensä' or 'their opinion'.