Macro is the building of units/structures/upgrades.
Players that have "good" Macro, will adjust so they are building the correct units/structures/upgrades to directly counter what the enemy is doing. They will also often be using most if not all of their resources constantly.
Never store minerals/gas unless you are saving up for a burst of a certain unit type. For example, Protoss will save minerals once their 4 Warp Gates are complete, so they can spend 5-600 minerals on some stalkers instantly.
Zerg will save as much gas as possible before their Spire is complete so they can build 6-7 Mutalisks straight away.
One rule that you must abide by... never stop making probes!!!
best way to do it is hotkey your Nexus to something like 4. then when your fighting or focusing on something else, you can just press "4e" and it will build you a probe
Micro is how you handle your individual units in and out of combat to make them more powerful.
This can be done by making use of the environment (cliffs, ramps etc..), or taking advantage of specific units abilities, Marines/Stalkers are very good at kiting melee units to deal more damage and allow them to survive for longer
Really good players are able to Macro AND Micro at the same time. You're a long way off this yet, it doesn't hur tto practise though !! I still can't do it, I always build the wrong units.. don't I Rob?