This attribute allows you to change the behavior of non-block tags to work as block tags, for example a <span> tag can behave like <div> Check on Google for better understanding of what it does, search for "difference between block and inline tags"
display block tag property is used in div. and this allow any div to display the iv as block , mainly w use this property to display to create the menu, where we keep the submit like buttns on left to right menu.it helps to keep its display like rectangle block.