It depends on the type of defence. Some items are percentage defence items, which means they defend a certain percentage of an icon type. For example, Downsize! blocks half of all icon types, or Ring of the Lost blocks all dark icons, regardless of your defence boost. Some other items have a fixed icon defence, such as Leaf Shield -- it defends against 5 earth, 5 water and 3 physical icons. Defence boosts work (pretty much) like
strength boosts, so if a 300-boost player and a 55-boost player both use a leaf shield against the same opponent (say, a 125-booster), the 300-booster will block a lot more damage. (Also, yes, if that 300-booster fights a much weaker opponent, the screen will show a lot more blocked icons than there actually are -- this is why you have to ignore shown icon amounts and look at the actual numbers.)
That's basically it... I mean, there's more to it (stance always complicates things), and maybe someone else who knows more than I do will come along and explain in detail, but I think that's all you really need to know.