This is the 4843rd most frequent Turkish word.
eklendiğini
"That it is added" or "has been added."
This usage of 'eklendiğini' shows it in a relative clause where it's used to describe having been added to the lesson.
Derse yeni bir konu eklendiğini duyduk.
We heard that a new topic was added to the lesson.
In this sentence, 'eklendiğini' conveys the notice of the addition of a new feature to the system.
Sisteme yeni bir özelliğin eklendiğini fark ettim.
I noticed that a new feature was added to the system.
Here, 'eklendiğini' is used to relay the statement of additional annotations to the document.