Fewer motors than degrees of freedom
A system's degrees of freedom (DOF) are the independent ways it can move: a robot arm's joints, a drone's position and orientation, a walking robot's limbs and torso. A fully-actuated system has an independent motor, or equivalent control input, for every one of those degrees of freedom, so at any instant a controller can push each DOF directly wherever it wants. An underactuated system has strictly fewer control inputs than degrees of freedom, which means some directions of motion can only be influenced indirectly, through the physics that couples them to the parts that are actuated.
A pendulum on a cart is the classic example: the cart can be pushed left or right, but the pendulum's swing angle has no motor of its own. The only way to control the pendulum is through the dynamic coupling between cart motion and pendulum motion. That coupling is not a design flaw to be removed, it's the entire control problem.