Swing Weight (Iron)

hi hi... can some1 help to explain what is Swing Weight, of D1, D2, D3, etc... mean ?

Am a beginner and hving a Mizuno Iron set now.... somehow I realised the head weight is light as I compare with the Titleist wedges of mentioned D4 D5. As a beginner, maybe I am confuse but I am quite comfortable with the Titleist wedges as I can felt the weight during downswing and more controllable..... (I am kind of a Gym guy lor) Does it mean I am more suitable for heavier head iron/Hybrid/FW/Driver ?

Think I overhear some saying that Mizuno iron are tend to be head weight light....