This is the 2417th most frequent Turkish word.
hepsinin
"Of all of them" or "all of theirs."
The word 'hepsinin' here is used to mean 'of all of them' in possessive form, referencing their achievements.
Hepsinin başarılarını kutladık.
We celebrated all of their achievements.
In this sentence, 'hepsinin' is used to indicate possession, meaning 'all of theirs' relating to their efforts.
Arkadaşlarım yeterince çalıştı ve hepsinin emeği çok değerli.
My friends worked hard, and all of their efforts are very valuable.
Here, 'hepsinin' means 'of everyone,' showing collective possessiveness regarding the target.