This is the 957th most frequent Turkish word.
aldıkları
"The things they take/receive."
Here, 'aldıkları' is used as a relative clause meaning 'that they took', referring to 'lessons'.
Aldıkları dersler çok zordu.
The courses they took were very difficult.
In this sentence, 'aldıkları' serves as a relative clause meaning 'that they received', referring to 'gift'.
Dün aldıkları hediye onları çok mutlu etti.
The gift they received yesterday made them very happy.
Here, 'aldıkları' acts as a relative clause meaning 'that they made', referring to 'decision'.