This is the 928th most frequent Danish word.
overraskende
Surprising.
Here, "overraskende" is used to express surprise about an unexpected result, namely him winning the race.
Det var overraskende, at han vandt løbet.
It was surprising that he won the race.
In this sentence, "overraskende" describes the manner in which she appeared at the party; her arrival was unexpected.
Hun dukkede overraskende op til festen uden varsel.
She surprisingly showed up to the party without warning.
Here, "overraskende" conveys the unexpected speed of his ability to learn Danish.