gsap.defaults allows you to set attributes that will be inherited by every tween and timeline (when applicable) unless overridden by that tween or timeline. Any general settings that should not be inherited by individual tweens should go in gsap.config() instead.

For example, if you want to change the default ease of all tweens and timelines and the duration of all tweens you'd use: gsap.defaults({ease:"Power2.easeIn", duration:1});.