It's an analogy. Look up the concept of "thermal inertia".
Think of a concept closer to home: a hotplate on an electric stove. When you turn the power on, the electric coil (which has very little mass) gets hot very fast. Put a heavy saucepan on top, and it will heat up much more slowly, pulling the temperature of the coil down (or at least reducing its rate of temperature increase) as it does so. Increase the power to the coil, and its temperature will rapidly increase, while the saucepan follows along much more slowly. Long after the coil's temperature has plateaued, the saucepan will continue to heat up.
So it is with global warming. The atmosphere is the lightweight element that responds quickly to changes in energy balance, while the ocean is the saucepan that follows along ever further behind.
