This is the 1687th most frequent Turkish word.
şeyin
"Şeyin" means "of the thing" or "the thing's" in English.
Here, 'şeyin' is used to indicate possession of 'şey' (thing), meaning 'of the thing'.
Bu şeyin ne olduğunu anlamıyorum.
I don't understand what this thing is.
In this instance, 'şeyin' is used to convey possession referring to 'bir şey' (something), signifying 'of something'.
Bir şeyin dikkatini çektiğini fark ettim.
I noticed that something caught your attention.
Here, 'şeyin' means 'of that thing,' describing the mystery related to it.