Omitting the parameter returns the current value (getter), whereas defining the parameter sets the value (setter) and returns the instance itself for easier chaining.
Version:
GreenSock Docs
Tween
.totalProgress()
.totalProgress( value:Number, suppressEvents:Boolean ) : *
[override] Gets or sets the tween's totalProgress which is a value between 0 and 1 indicating the position of the virtual playhead (including repeats) where 0 is at the beginning, 0.5 is halfway complete, and 1 is complete.
Parameters
value: Number
(default = NaN
) — Omitting the parameter returns the current value (getter), whereas defining the parameter sets the value (setter) and returns the instance itself for easier chaining.
suppressEvents: Boolean
(default = false
) — If true
, no events or callbacks will be triggered when the playhead moves to the new position.
Returns : *

Details
[override] Gets or sets the tween's totalProgress which is a value between 0 and 1 indicating the position of the virtual playhead (including repeats) where 0 is at the beginning, 0.5 is halfway complete, and 1 is complete.