This is the 3767th most frequent Russian word.
чему
The Russian word "чему" means "to what" or "to which" (dative case).
The word 'чему' introduces the object of the delight or happiness, indicating an indirect question.
Я не понимаю, чему ты радуешься.
I don't understand what you're happy about.
Here, 'чему' is used to specify the learning or knowledge being described.
Объясни мне, чему ты научился на этом курсе.
Explain to me what you learned in this course.
In this sentence, 'чему' introduces the subject matter that sports teach.