I was asked this question recently and also saw it being asked on the Fortinet Support Forums as well. This is really a simple question to answer though. Policy ID 0 is the default policy (the implicit deny) that comes by default on the FortiGate. They also come with an explicit allow right above it now which helps people utilize the device with no configuration right out of the box. FortiGate devices used to be deny by default on first use so that you had to allow the traffic you wanted. Either way, it is the deny traffic policy that comes built into the FortiGate.