First things first: There are three extremely important skills that every player needs: Macro, Micro and APM.
Micromanagement determines how well you can move small groups of units, single units, parts of your army and how well you use the unit’s special abilities. A small army can beat a larger army when the player has the superior micro ability.
APM on the other hand describes your Action Per Minute – the number of times you click with your mouse or press any button. It basically means how much commands you can give: More Commands result in more units, more expansions, better micro and macro.
Now, Macro-management is about how well you can keep track of the “big picture”. Instead of controlling single units it’s about controlling all units and buildings at once.
Let’s take the Zerg as an example: Zerg players have to produce Drones and other units, inject new Larvae constantly, get new Overlords in order to prevent a supply-block and also expand their bases. And at the same time you must control your army or it will be out of position at a critical time. And there’s more: You have to prevent a supply-block by spawning Overlords when your supply-count demands it and build tech building for upgrades.
You have to be a pro in multitasking to keep track of every task that comes up. If your macro skill is poor you will probably only focus on one of the tasks at a time. Of course many tasks would never be done this way. If you want to have control over every unit in your army and every building, you need superior Macro-management.
Here’s a way to improve your macro: Save your own replays and analyse them later on. Check if you prepare all the tech buildings in advance or if you get supply blocked at any time.