Macro is everything you do to have more than your opponent: workers, bases, production and upgrades. From Bronze to Diamond, most games are won here, not in micro.
The cycle
Think of a short cycle you repeat all game: workers, supply, production, spending. Every time you finish an army action, go back to the cycle.
One metric
Watch your unspent resources. If they stay above 500 for long, your cycle stopped. After each game, check the resources graph in the replay and find where they piled up.
To practice
Open the Macro without pauses routine in the Build Lab and play one game against the AI with a single goal: production never stops.