A sentence adverbial - banal, but there it is.
However, to make it a bit more complicated .... Sentence adverbials may be :
Just an adverb - Frankly, he hasn't got a chance.
Prepositional phrases - In all frankness, he hasn't
Infinitive clauses - To put it frankly, he .....
-ing participle clauses - Frankly speaking ...
Past participle clauses - Put frankly, he .......
Finite verb clauses - If I may be frank, he ....
They may come in front, mid or end position. Consider also ... He hasn't, frankly, got a chance.
He hasn't got a chance, quite frankly.
Front is probably most usual though.
A sentence adverbial differs from other adverbials in that they are often a comment on the proposition expressed - hence the commas which detach them from the rest of the sentence. Compare those above with the following, in which frankly
is just an adverb of manner modifying the verb - He spoke about it very frankly.
OK, OK more than you wanted to know. But I thought a one-line answer was too easy
PS Examples from A Communicative Grammar of English
, Leech and Svartvik (Longman)