A tween is when one move transitions into another one. It's similar but distinct from a cancel since the second move comes out as if it were started when the first move was, rather than when its input was done. It can also refer to tweening in the normal sense of blending two animations together.
Ordinary tweening occurs constantly as basic animations such as those for backdash, sidestep, guard, and crouch all blend into the startup animations of other moves. However, tweening is not merely an aesthetic effect in Tekken as it can change the game logic itself by altering hurtboxes and states. Also, by changing the visuals it can make it harder for opponents to react to moves. For example, a throw done from a sidestep can look a bit different than normal and be trickier to break.
The term is from animation referring to the frames between two keyframes. These are “in between frames” of animation that were never specifically created but rather generated automatically by an algorithm. As a verb it refers to transitioning from one keyframe to another.