This is the 367th most frequent Serbian word.
њихов
"Their" (possessive pronoun).
The word 'њихов' is used here as a possessive pronoun to indicate possession by a third person plural (their advice).
Њихов савет је био веома користан.
Their advice was very helpful.
Here, 'њихов' denotes possession of the car by them (third person plural).
Узео је њихов аутомобил без питања.
He took their car without asking.
In this context, 'њихов' again indicates possession, referring to the project that belongs to them.