thats your master security lock........ basically wehn you go to the phone settings to change some of the main stuff (for instance ##778#) you will have a screen that says view, edit, and cancle..... if you click on edit it will ask you for a passcode....... you enter your msl number there and than you can edit certain things....... this si used to activate your phone as well as to do my favorite tweak that i do to any ppc and that is change the sci (slot cycle index) what the slot cycle index does is change the time that it takes from when it starts to ring on the other persons side before it rings on yours..... its set to 2 which is 2 rings that the caller heres before your phone rings........... i change mine to 0 and i get the calls immediately. and the best part is that there is no battery drain even though you are setting it to constantly search for new towers non stop instead of once every 2 seconds.