This is the 1913th most frequent Turkish word.
iyisini
"The better one" or "the good one."
Here, "iyisini" refers to selecting the best option.
Bu iş için iyisini seçmeliyiz.
We must choose the best one for this job.
In this sentence, "iyisini" refers to choosing individuals of better quality from a group.
Elmaların iyisini ayıralım, diğerlerini de komşuya verelim.
Let's pick the better apples and give the rest to the neighbor.
Here, "iyisini" is used to indicate selecting the better choice from a selection.