toggleActions

By default, ScrollTrigger simply plays the linked animation when the start scroll position is hit, but it can perform any of the following actions at any of the four toggle points: play, pause, resume, reverse, complete, restart, reset, or none.

This line's animation will play when it enters the viewport, and then take no other actions after that. It's the default toggleActions: "play none none none"

This line's animation will restart when it enters the viewport then pause when it leaves, resume when scrolled back into view, and finally pause when it leaves again (scrolled all the way back). toggleActions: "restart pause resume pause"

This line's animation will play when it enters the viewport, complete when it leaves, reverse when you scroll it backwards into the viewport again, and reset when you go all the way back past the beginning. toggleActions: "play complete reverse reset"

This line's animation will play when its top edge hits the center of the viewport (start: "top center"), then it will pause 100px from the top of the viewport (end: "top 100px"). Scrolling backward past that point will resume, and scrolling all the way back past center will pause again. The tween also has an infinite repeat.
toggleActions: "play pause resume pause"

DONE!