Key ability: Constitution
Concentration represents the ability to maintain focus during distractions, pain, danger, and other interruptions.
A Concentration check is required only when a rule identifies a disruption to casting; routine spellcasting does not require one. The rule that causes the disruption sets the check’s DC. Unless that rule says otherwise, failing the check causes the spell to fail and its casting resource to be expended: a spell slot for a slot-based caster or the Sorcery Points committed to the spell for a Sorcerer. Current examples include casting while mounted or entangled; see Spells for these interactions.