Reassuringly is the adverb: you do something in a way that makes people feel less worried or frightened. She smiled reassuringly. He spoke reassuringly to the child. Writers also put it at the front of a sentence to comment on the whole fact: “Reassuringly, the door was still locked.” That means “this next thing should make you feel safer.” Though actually, it is more at home in writing than in quick talk. In conversation people often just smile and say “Don’t worry.” If you already know the adjective reassuring, this word is that idea turned into “in a reassuring way.” It does not add a new meaning; it only changes how the word sits in the sentence.

令人安心地、讓人放心地