Really, Truly
adverb
本当 is reality or truth (level 5 vocab). If you add on に, this changes the word to an adverb, and we get really or truly. 本当に!
ほんとうに
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. It shares the reading with the level 5 本当 as well, but just has a particle on the end. Really!
この石ころ、本当に大切？
Is this stone really important?
この地図が本当に正しいか不安です。
I'm worried if this map is truly accurate.
この車、本当に中古なんですか？
Is this car seriously used?