This is the 77th most frequent Swedish word.
helt
"Completely," "entirely," or "totally."
Here, 'helt' is used as an adverb to intensify the adjective 'säker', meaning 'completely'.
Jag är helt säker på mitt val.
I am completely sure about my choice.
Here, 'helt' is used to emphasize the adverb 'ensam', stressing that the subject was 'completely' alone.
Hon åt upp hela kakan helt ensam.
She ate the entire cake completely alone.
In this sentence, 'helt' functions as an intensifier for the adjective 'underbar', emphasizing the titular quality as 'completely' wonderful.