Reflashing... so what is it and why bother?  Simply put, it is installing new software onto a writeable solid state microchip.  Just like  when microsoft notifies you that you need a critical update for your windows operating system and you proceed to download it and then install it to your harddrive, automobile manufacturers have similar updates.  Now it's not as streamlined as updating windows but the process is fairly similar.  The first car I ever flashed was actually not a car but my home computer.  Anyone who is into computers has flashed their bios at some point and the reasons for doing so could be numerous adding new features improving performance or perhaps even using a custom flash for overclocking the processor.  Whatever the reason, there is always some benefit.  You may not always notice somehing and you are not always told everything that has been changed but you know that an engineer isn't going to tweak something if they haven't calculated that this was going to make things run better.  So, when it comes to cars,  the way the procedure goes is... we hook up our scan tool to your car and retrieve the part number out of the PCM, then we look up that part number on the manufacturers website and that will tell us if there is any new revisions of the software or more correctly termed firmware.  If it shows there is an update, we then need to purchase a subscription and download the manufacturers flashing software and the update.  The society of automotive engineers came up with the J2534-1 standard which describes in detail the protocols to be used when communicating with the car's computer.  The items required to flash your car's computer are a computer with internet access, a subscription to whichever manufacturerer you want to flash (ie: General Motors, Ford, Chrysler),  a J2534 passthrough device which acts as the middle man between the car's computer and the PC.   The reflash software is then started and away we go.  Approximately 30-45 minutes later, the process is done and your computer is up to date.  So, as the end user will you notice any difference?  I have had some vehicles that I have worked on that after the flash was done, there was no perceived change in the operation although the note that came with the update said it addressed problems with a code that was setting .The computer was calibrated in such a way that it was too sensitive and the flash corrected that.  I've had other vehicles come in where the difference in the way the car ran after the flash was unbelievable. It was like I was driving a different car!  To conclude,  reflashing is a lot like microsoft having an update that needs to be installed and all you have been told is that it is a security update.  You may not know  the changes that are taking place with the update but they are definitely there. You do not hesitate to install the update because you trust it will be an improvement.  A car update works the same way. The automotive technician will let you know if an update is needed to resolve problems in your vehicle or if an improved software update could possibly improve fuel economy in your vehicle.