Normally, the tooltip when you mouse over a macro will be just the name of the macro. #showtooltip makes it show the tooltip for the actual spell/item. The game will try to pick the best spell/item to show, and for many macros it will show what you want, and just a plain #showtooltip is all you need. You can force the game to use a specific spell/item by adding that after the #showtooltip. This also affects the icon and status (reagent counts, cooldowns, etc) on the button.