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.