The term ‘IWB’ stands for Inside the Waist Band type of holsters. These are holsters designed to to fit inside the waist band of your pants and conceal your handgun under your over-shirt. These are great holsters for conceal carry with a ultra compact to midsize handguns. Some even allow for you to tuck your shirt in around the holster with only the belt clip exposed.
The secret to comfortably wearing an IWB holster is to wear a thick and sturdy belt and to have pants or shorts and the belt that are about a size bigger than you need.
On a weird note, this type of carry for a chubby guy like me works well when worn somewhere between 2 o’clock and 4 o’clock. Something about my side belly roll helps cover the grip of my gun thus concealing it even more. This type of carry method also works for the popular appendix carry if you favor that position.