The adverb "truly' means "very much" or "honestly". You can use it before an adjective like this:
I'm truly sorry for what happened.
The most common adjectives that follow "truly" are:
- truly great
- truly remarkable
- truly amazing
- truly sorry
- truly happy
You can also use "truly" with a verb:
I truly forgot all about it.
I truly believe that you will achieve your dream some day.
"Truly" is pretty formal.