This is the 12th most frequent Greek word.
ότι
"Ότι" means "that" or "because" in English.
In this sentence, "ότι" is used to connect the main clause with the direct object, explaining what was understood.
Κατάλαβα ότι έκανες λάθος.
I understood that you made a mistake.
Here "ότι" serves to introduce the content of what was told in the main sentence, functioning as a conjunction to connect clauses.
Σου είπα ότι είμαι κουρασμένος.
I told you that I am tired.
Here, "ότι" is used to introduce a subordinate clause that provides additional information, in this case, the subject of the belief.