On its own, "Lui aggiunge" is not much of a sentence.
Reverso is a helpful website for verbs in context. An example given:
"Lei aggiunge tensione ed emozione ad una situazione già difficile."
She adds tension and emotion to an already difficult situation.
"Lui aggiunge qualcosa al loro calendario."
He adds something to their calendar.
there are people discussing the pros and cons of an upcoming move. one person, let's call him larry, has been very quiet the entire discussion but has made some valid points. someone calls him out saying that he isn't adding anything to the conversation, when someone stands up for him-- "he adds!"