This is an adverb, but it has the same meaning (basically) as the kanji. It means certainly, and it can also mean surely.
The reading for this one is certainly weird. To remember the reading for this word, just think how you're certain that if you stash the treasure, nobody will find it here.
I'm certain I saw Koichi fly about one hundred meters with my own eyes.
It seems like it might be difficult to operate this machine.
Tofugu has certainly delivered a one-two punch to the traditional Japanese language learning industry.