Short answers, pulled from the story.
Inbetweening is the process of creating intermediate frames, called inbetweens, between two keyframes in animation. The goal is to produce the illusion of smooth movement by transitioning one image into another. It applies to both traditional pencil-and-paper animation and modern digital animation.
Dick Huemer developed the inbetweening system in the 1920s. Art Davis is credited as the first inbetweener, meaning the first person to hold that role as a dedicated position in an animation studio.
Animating on twos means producing one drawing for every two frames, resulting in 12 drawings per second rather than 24. This technique has been standard practice for over 100 years and was used in Fantasmagorie (1908), widely considered the first fully animated movie.
Nestor Burtnyk and Marceli Wein of the National Research Council of Canada received a Technical Achievement Academy Award in 1997. They were honored for pioneering work in the development of software techniques for computer-assisted keyframing for character animation.
Adobe Flash and Animo (developed by Cambridge Animation Systems) were among the earliest programs to use automatic interpolation for digital tweening, appearing in the late 1990s. Tweenmaker was released around 2006, and the free program Synfig specializes specifically in automated tweening.
Ease-in and ease-out are mechanisms that define the physics of a transition between two animation states. An ease-in transition starts slowly and accelerates; ease-out starts quickly and slows down. Both control the linearity of a tween in digital animation software.