This is the 303rd most frequent Slovenian word.
njihova
"Their" (possessive pronoun).
The word 'njihova' is used as a possessive pronoun to describe that the house belongs to them (a group of people).
Njihova hiša je zelo velika.
Their house is very big.
Here 'njihova' again denotes possession, explaining that the cat belongs to them (a group or number of people).
Nisem vedel, da je to njihova mačka.
I did not know that this is their cat.
In this sentence, 'njihova' is used possessively to describe that the decision pertains to them (a group of people).