Initiative is the measure of your reaction speed, mentally and physically. You roll initiative for your mind(Based on your wit) and initiative for your body(Based on your reflexes).
When in combat or performing an action where the reaction speed of you or others is important Initiative is used to find out when your mind and when your body react. Mental initiative is based off of wit.(1d100+5xwit) And physical initiative is based off of reflexes.(1d100+5xreflexes)
How to use these initiative values is governed by Combat Rules.