From Nordic Larp Wiki
The use of Safewords is a common feature in many larps to ensure player safety and prevent situations from getting out of hand. They are non-diegetic signals to make the other players act in a certain way according to which safeword is used.
Even though safewords have been a part of larp rule systems since the 90s people seem to have a high threshold to actually use them. This has lead to questioning of how effective they really are. This was one of the themes of Solmukohta 2012 panel debate "The Great Player Safety Controversy Panel".