Orbiters are devices designed to send their victims so far into the sky that it would take hours for them to fall back down in the normal way and/or their coordinates no longer fit within SL’s world model (hence the name "orbiter", because it sends you into orbit). In practice Second Life today has limitations that reduce the effectiveness of orbiters. If you are being orbited, teleporting away or – in extreme cases – relogging will fix being orbited.
There exists a 4096m hard limit on the height an object can move (or be moved) to, which prevents orbiting from continuing above this height (as the orbiter object that surrounds and pushes the avatar will be unable to travel higher than this, meaning gravity takes over until the avatar drops back down below 4096m where the orbiter can again reach them). Although an avatar (and its attachments) can travel up to one million meters without ill effects, few if any of today's orbiters are powerful enough to throw somebody over five thousand meters in height.