Tweener has nothing to do with young people, basketball or that
tennis move between your legs.
Instead, Tweener is a tool similar to TweenMachine or aTools/animBot. It allows you to quickly create in-betweens or adjust existing keys by interpolating towards adjacent keyframes, and can speed-up your workflow when creating breakdowns and in-betweens. I am an animator myself and so I very much tailor the tool to my own needs.