T-boning can be a t-shaped pin. It can have the effect of the robot being t-boned moved sideways, or it can be against a wall.
T-Boning is generally creating bumper-to bumper contact with another robot with the front end of your robot pushing the other robot perpendicular to that robot's direction of motion.
The effect it has is often preventing motion in the desired direction, and thereby keeping the affected robot from scoring.
This can be better explained by
someone smarter than me.
You can see it in most FRC matches this year that had effective defense.