Kitabı okudum.
English: I read the book.
The 'belirtili nesne' in Turkish is the 'definite object' in a sentence. It is used when talking about a specific thing or person that receives the action.
Use the 'belirtili nesne' when you are talking about a specific object or person that is affected by the verb. For example, when you say 'I see the cat' (a specific cat), you use this form.
Kitabı okudum.
English: I read the book.
Arabayı yıkadın.
English: You washed the car.
Elmayı yiyor.
English: He/She is eating the apple.
Köpeği gördük.
English: We saw the dog.