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 ... etc
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)