Macros

Macros means "Macronutrients".

Macronutrients are the foodstuffs you need in large quantities for you body to function.

Carbohydrates (which includes sugars and fiber)
Fats
Proteins

Sometimes you'll see people talking about percentages of macros or "hitting their macros".

You can set macros based on all sorts of different philosophies. A normal low fat diet might look like 50% carbs, 30% protein, and 20% fats. A low carb diet might look like 30% carbs, 35% protein, 35% fat. I believe the "Zone" diet looks like 40/30/30. Other people may split it evenly such as 33/33/33.

Hope that helps.