A TweenLite instance (with a duration of 0) which can optionally be inserted into a TimelineLite/Max instance (although it’s typically more concise to just use the timeline’s set()
method).
GreenSock Docs
TweenLite.set()
[static] Immediately sets properties of the target accordingly - essentially a zero-duration to() tween with a more intuitive name.
Parameters
target: Object
Target object (or array of objects or selector text) whose properties will be affected.
vars: Object
An object defining the value for each property that should be set. For example, to set mc.x
to 100 and mc.y
to 200, do this: TweenLite.set(mc, {x:100, y:200});
You may also define any of the special properties baked into GSAP like delay, onComplete, etc.
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Returns : TweenLite

Details
Immediately sets properties of the target accordingly - essentially a zero-duration to() tween with a more intuitive name. So the following lines produce identical results:
TweenLite.set(element, {x:100, y:50, opacity:0});
TweenLite.to(element, 0, {x:100, y:50, opacity:0});
And of course you can use an array to set the properties of multiple targets at the same time, like:
TweenLite.set([obj1, obj2, obj3], {x:100, y:50, opacity:0});
Or use selector text to target DOM elements that match that criteria.