This is the 877th most frequent Persian word.
ی
The Persian letter "ی" (Yeh) represents the sounds "y" (as in "yes") or the vowel "ī" (as in "see") and serves as a suffix for possessive pronouns or noun pluralization.
In this sentence, 'ی' serves as a suffix to 'خوب' (good) to turn it into 'خوبی' (good, qualitative), implying the possession or the qualitative attribute of good, forming an adjective.
کتاب خوبی خواندم.
I read a good book.
In this sentence, 'ی' is connected to 'بعد' (next) as 'بعدی' (following/next), which is used as an adjective qualifying 'station' (ایستگاه), indicating sequence.
ایستگاه بعدی کجاست؟
Where is the next station?
Here, 'ی' is part of the noun 'دوستی' (friendship), and it is used to convey the abstract concept of companionship or friendship, transforming the root 'دوست' (friend) into the noun.