This is the 11974th most frequent Turkish word.
gitmeleri
"Gitmeleri" means "their going" or "the act of them going."
Here, 'gitmeleri' refers to their action of leaving ('gitmek'), conjugated to indicate necessity for the group.
Onların buradan gitmeleri gerekiyor.
They need to leave from here.
In this sentence, 'gitmeleri' refers to the theoretical action of them leaving ('gitmek'), emphasizing a hypothetical scenario.
Gitmeleri çok iyi olurdu.
It would be very good if they left.
'Gitmelerini' represents the action of leaving ('gitmek') conjugated in the accusative case to indicate the object of wishing.