This is the 650th most frequent Czech word.
čím
The Czech word "čím" means "with what" or "by what" (instrumental form of "co").
The word 'čím' indicates the instrumental usage of 'what' referring to a profession or role.
Čím chceš být, až vyrosteš?
What do you want to be when you grow up?
Here, 'čím' is part of a comparative phrase expressing a proportional relationship.
Čím více čtu, tím více rozumím.
The more I read, the more I understand.
In this case, 'čím' introduces the instrumental object of the action.