Regular Fan Speed is the speed at which the fans on the toolhead will spin, unless the layer is very small. It applies to the layers whose printing time is not smaller than the Regular/Maximum Fan Speed Threshold.
A greater speed allows better cooling. This reduces oozing,the pillowing effect and stringing，and it can produce better overhangs.
A lower speed will reduce warping with some materials and will make the print stronger.
The fan speed should almost always be maximum when printing materials with a low glass transition temperature range, such as PLA.