This is the 14684th most frequent Turkish word.
projlerini
"Projelerini" means "their projects" in English.
In this sentence, 'projlerini' is used in its accusative plural form referring to the projects owned by 'Arkadaşım' (my friend).
Arkadaşım projlerini bitirdi ve sunum için hazırlanmaya başladı.
My friend finished his/her projects and began preparing for the presentation.
Here, 'projlerini' is again in the accusative plural form, indicating that the files are related to his/her projects.
Projlerini öğrenmek için dosyalarını kontrol etmelisin.
To learn about his/her projects, you should check their files.
In this example, 'projlerini' reflects the projects being submitted, owned by the subject of the sentence, and assumed as plural.