Diaphragmatic breathing is a practice of conscious belly breathing that builds core strength and helps to reduce anxiety.
For passive diaphragmatic breathing, place one hand on your belly and the other on your chest. Focus on the movement of your abdomen as you breathe in and out, and actively breathe "into" the belly while inhaling, allowing your stomach to expand. During this expansive breathing exercise, relax the muscles of your ribcage and spine.
For active diaphragmatic breathing, you're going to be working on core strength. By making the breath stronger than in the passive breathing and adding in a sound through pursed lips during your exhale ("Shhh" or "Ssss"), you'll be strengthening your core.