This is the 552nd most frequent Czech word.
čem
"Čem" is the instrumental form of "co," meaning "what" in English.
Here, 'čem' is used as a part of the ovjective clause referring to the topic of conversation.
Nevím, o čem mluvíš.
I don't know what you are talking about.
In this sentence, 'čem' is employed to indicate a specific issue being discussed or referenced.
Vím, v čem je problém.
I know what the problem is.
The word 'čem' here signifies the subject of someone's thoughts or considerations.