The verb "to ghost" is a new slang use of this word. It means to stop responding so that it seems like the person just disappeared (like a ghost).

Agree with the other answer. It's mainly used when talking about relationships, if someone stops all contact with you (the implication that they are also avoiding you usually without explanation) then you have been "ghosted" or they are "ghosting you". So in the phrase "I'm sorry I ghosted" it basically means "I'm sorry I ignored you".