Simply put, macronutrients are what make up the caloric content of the food we eat. There are three: protein, carbohydrates, and fat. Each blueprint provides you specific macronutrient ratios and caloric needs to help you meet your goals.
The primary focus is to implement an effective (not to be mistaken with unhealthy) caloric deficit. Loss can only occur through a negative calorie balance, whereby you expend more calories than you consume.
Maintaining body composition occurs upon eating at a caloric balance - consuming as much energy as you expend.
Eating at a caloric surplus should be at the forefront when aiming to gain mass, paired with a regular strength-training routine.