Simplest explanation is visual: hard flood you see the image of the screen after the flood, soft you only see a film of ink across the screen. This is looking at the squeegee side, not the shirt side, just to be clear.  More technically speaking, with a hard flood you are filling the gasket created by the stencil by pushing the ink into it.  With a soft flood you are covering the image area with ink, but not the gasket of the stencil.  Soft flooding helps A LOT when printing waterbased inks, but really is only used in plastisol to minimize dot gain when other variables don't allow you to print with minimal pressure.  I soft flood pretty much everything but low mesh white screens, which have high eom.