style disjuncts

When a speaker exploits a style adverb, s/he is indicating in what mode the sentence is being uttered, i.e., the speaker is being frank, truthful, honest, etc.

Confidentially, I can't stand my boss.
I honestly can't help you.
Seriously, don't tell anyone.

Note that if it is put at the end of the sentence, it may become an adverb of manner.

I didn't speak to him honestly./ Honestly, I don't speak to him.