What is diaphragmatic breathing? And why is this important in fitness?

What is diaphragmatic breathing? And why is this important in fitness?

Diaphragmatic breathing, or deep breathing, is breathing done by contracting the diaphragm, a muscle located horizontally between the thoracic cavity and abdominal cavity.

Respiratory muscle training (RMT) promotes diaphragmatic breathing.
RMT improves exercise-limiting shortness of breath and delays time to fatigue in healthy people and athletes.
RMT enhances performance by improving respiratory muscle endurance, maximal sustainable ventilatory capacity (MSVC) and maximal voluntary ventilation (MVV).