Geometry versus force
Kinematics, which you covered in Robot Kinematics & Motion, describes where a robot's joints and links are and how they move, purely in terms of positions, velocities, and angles. It never asks what caused that motion. If you specify that a joint should sweep from 0 to 90 degrees in one second, kinematics can tell you the resulting path of every link along the way, but it has nothing to say about whether the robot's motor is actually strong enough to make that happen.
Dynamics is the layer underneath that answers the harder question, what forces and torques are needed to produce that motion, given the robot's mass, the distribution of that mass along each link, gravity pulling on every joint, and friction resisting movement. A robot arm can look completely correct kinematically, the planned path is smooth and reaches the target, and still fail in the real world because the torque required to follow that path exceeds what the motors can deliver, or because the plan never accounted for gravity trying to pull the arm down mid-motion.