A Lorn detachment is a group of soldiers that, for whatever reason, is not expected to return alive. Most often this is because they have been sent to aid an ally in dire trouble although it can also be because the mission is so incredibly dangerous that it would not be attempted save for its great importance. They are frequently made up of volunteers.
When sent to allies it is intended as a gesture of solidarity. How it is perceived can very much depend upon what was expected: help when none was asked is seen as noble; a Lorn detachment when a defensive treaty is in place can be an insult. Often there is reason why a full allied force cannot be sent, although that is of little comfort to those in need.
As a note, it is virtually always capitalised in the books, so there may be some sort of story behind it.