Ability to maintain a constant air stream for a stable tone — breath support (appoggio) is the foundation of singing.
Exercise: inhale to the belly, place hands on belly. On exhale 's,' keep belly firm. Pressure goes up to the chest, not out.
Without breath support, there's no stable tone, dynamics, or endurance. The voice shakes, the throat tightens, it tires quickly.
Opera singers train breath support for years. 'Messa di voce' — crescendo and decrescendo on one note.