"Hold notes are the sliders and spinners of osu!mania. When the hold note reaches the judgement line, tap the starting note in time with correct key, hold, and release it at the ending note of the hold note.
Health bar regeneration occurs slowly while holding the hold note(s). Depending on the song and mapping quirks, it is possible that other notes or hold notes to appear while holding the previous hold note(s)."
That is literally what is on the wiki.