I take it to mean that the horse is wrong in the wither, for some reason or another - one could think that the vertebra have misalined, and come out of line, but there is a school of thought that thinks that the horse vertebra can't move sideways, and if it did then it would take more than a human to put it back into place.
For me, it means that the horse is slightymisalined in front of the saddle - caused by any number of reasons, slip in field, roll on a stone etc.
I would expect my horse to struggle to work evenly up through both shoulders if misalined, to lean more one way and to be slighty heavier through the rein than normal. Usually this type of misalinment goes with another - perhaps under or behind the saddle or at the poll. As if the horse has been compensating for a while.
Are you asking because you want to know, or because you already know?
MASSIVE pet hate of mine..... I seriously wish "back folk" would stop using the term "out" when it comes to bones.... horse owners are NOT stupid and would understand more complex, correct terminology.