This is the 199th most frequent Czech word.
co
"What."
'Co' is used as an interrogative pronoun to ask a question about what someone is doing.
Co děláš?
What are you doing?
'Co' here functions as a subordinating pronoun introducing an indirect question about what occurred.
Nevím, co se stalo.
I do not know what happened.
'Co' is used as a relative pronoun to specify the content of 'everything' in this context.