Agility is the capactity (or readiness) to move with quick easy grace.
When we are prepared (with our knowledge, skills, confidence) we are more ready to face challenges and react to changing tides as we do so. Agility is the capacity to adapt to change, be it in markets, tech, team structure, etc.
Teams and people that display this behaviour have usually a carefree disposition that comes with the confidence that they will be able to master any challenges that come their way.