This is the 56th most frequent Czech word.
totiž
"Namely" or "that is to say."
used to introduce an explanation or clarification
Musím přestat, totiž už je pozdě.
I have to stop, as it is already late.
used to provide reasoning or justification
On nekoupil lístky, totiž zapomněl peníze doma.
He didn't buy tickets, as he forgot his money at home.
used to indicate hesitation before introducing additional information