With this exercise, one arm performs a push-up while the other slides out straight ahead. (You'll need a pair of sliders obviously. You can buy the ones sold to the fitness market or just pick up some furniture sliders on the cheap.)
Ab rollouts and push-ups both focus more on anterior core stability, but the unilateral element of this exercise challenges rotary core stability as well.
Don't worry if you can't extend out all the way – just go as far as you can while maintaining control of your core.