"Macros" is short for the word macronutrients. These nutrients are needed in larger amounts when compared to micronutrients (vitamins and minerals). The three macros we refer to and that Carbon adjusts are protein, carbs, and fat. Since these macros are a source of calories for your body, they ultimately dictate your overall energy balance and results.
Protein is essential because it provides us with the building blocks for the construction of all body proteins. For those exercising and breaking down muscle regularly, it's vital to have enough protein in your diet to help rebuild muscle tissue. For those dieting, it helps keep you more full and shifts the weight loss more towards body fat instead of muscle. It also helps you burn more calories when compared to carbs and fat because it takes more energy to digest, process, and utilize. Protein has 4 calories per gram.
Carbs are important because they provide glucose, one of the primary energy sources in the body. Red blood cells, your central nervous system, and the brain all run exclusively on glucose (though the brain can switch to ketones in periods of glucose deprivation). Carbs can be stored as glycogen in the liver and muscles to be used as an energy reserve during intense activity or fasting/calorie deficit periods. Carbs have 4 calories per gram.
Fat is essential because it makes up the membranes of all cells in your body. It's also a rich source of energy with unlimited storage capacity. Excess calories from fat can be stored as body fat and used as a fuel reserve during times of starvation/caloric deficit to provide energy for working tissues. Fat has over double the energy content of protein or carbs and provides 9 calories per gram.
The fourth macro
Another macro worth mentioning is alcohol or ethanol. Though not typically thought of as a macro, it does contain 7 calories per gram and can contribute a significant amount of calories to someone's diet. If you are going to consume alcohol, you need to understand how to track it accurately in Carbon. Here's an article to help explain how to do this.