Destealth means to remove Stealthed from a champion, effectively revealing it so its location is known and it can be attacked. Champions can destealth when an opposing champion with Detection is nearby, when it gets damaged, or when an opposing champion attempts to move onto a space it occupies.
Note that Loss of life effects do not destealth a champion. So for example, losing HP from Drive or Exertion will not destealth a champion.