This is the 677th most frequent Slovenian word.
njihovem
"Njihovem" means "their" (possessive, masculine/neuter, singular, dative/locative case) in English.
The word 'njihovem' here is the locative form, referring to possession and location.
Na njihovem dvorišču raste veliko drevo.
In their yard, a large tree grows.
In this usage, 'njihovem' describes possession related to the playground.
Smo se igrali na njihovem igrišču.
We played at their playground.
Here, 'njihovem' shows possession regarding the contribution.