This is the 3211th most frequent Turkish word.
fikirleri
"Ideas" or "thoughts."
In this sentence, 'fikirleri' acts as the plural possessive form of 'fikir' (ideas) implying connection to 'öğretmenin' (the teacher).
Öğretmenin fikirleri öğrencilere ilham verdi.
The teacher's ideas inspired the students.
Here, 'fikirleri' indicates possession and plurality, connected to 'onların' (their).
Onların fikirleri her zaman ilginçtir.
Their ideas are always interesting.
In this case, 'fikirleri' references multiple ideas ('tasarım fikirleri') in the context of a discussion, not necessarily possessive.