This is the 17328th most frequent Turkish word.
öğrenmesi
"The act of learning" or "learning (of something)."
In this sentence, "öğrenmesi" refers to something he must learn, serving as a noun derived from the verb "öğrenmek" with third-person possessive suffix and gerund form.
Onun öğrenmesi gereken çok şey var.
He has a lot to learn.
This use of "öğrenmesi" represents the act of learning, related to the thing learned, expressed as a noun signifying the process.
Öğrenmesi sabır gerektiriyor.
Learning it requires patience.
Here, "öğrenmesi" refers to the specific instance of him learning a language, expressed in a possessive gerundial form.