Tweens special EaselJS-related properties for things like saturation, contrast, tint, colorize, brightness, exposure, and hue which leverage EaselJS's ColorFilter and ColorMatrixFilter (see http://www.createjs.com/#!/EaselJS for more information about EaselJS). Of course you don't need the plugin to tween normal numeric properties of EaselJS objects (like
y), but some filters or effects require special manipulation which is what EaselPlugin is for. Currently it only handles special properties related to ColorFilter and ColorMatrixFilter, and it can tween the "frame" property of a MovieClip.
GreenSock's EaselPlugin exposes convenient properties that aren't a part of EaselJS's API like
"tint", "tintAmount", "exposure", and
"brightness" for ColorFilter, as well as
"saturation", "hue", "contrast", "colorize", and
"colorizeAmount" for ColorMatrixFilter.
Learn more in the EaselPlugin documentation.