Initial Layer Flow adjusts the amount of material being pushed through the nozzle during the printing of the initial layer.
The Initial Layer Flow is normally set larger than the flow of other layers. This allows the material to be pushed harder onto the build plate, and thus improves build plate adhesion.
This setting can also be used to compensate for an improperly levelled heated bed. If the heated bed is too close, you'll want to reduce the flow. If the heated bed is too far, you'll want to increase the flow. Nevertheless, it's still better to properly level the heated bed at the beginning.