This is the 246th most frequent Czech word.
on
"He" (third-person singular masculine pronoun).
Here, 'on' is a personal pronoun meaning 'he', used as the subject of the sentence.
On jde do školy.
He is going to school.
'On' functions as a personal pronoun meaning 'he', indicating the person referred to earlier, and is used as the subject in the subordinate clause.
Myslím na to, protože on mi to řekl.
I am thinking about it, because he told me that.
The word 'on' is used at the beginning as the subject pronoun 'he', referring directly to a specific person discussed.