The shape of a wing is key to producing lift.
So it is for a sail because, as you may have heard, the sail is a wing.
Unlike airplane wings, conventional sails are made of flexible materials, because it’s lighter and it’s nice to be able to fold them up when not needed.
Sails have the requisite shape built into the fabric, but without a rigid structure to hold it there, the airflow isn’t happy, and without happy airflow, the sails won’t generate lift, and the boat will not move. Luckily, the airflow itself does a fantastic job of pressing against the sail and causing it to take shape. With the form in place, smooth airflow can be achieved, and off we go.
So convenient. And this system works very well. Usually.
What if the wind is very light, too light to press the sails into shape? See, the sail material has mass to it and some amount of rigidity. Against strong winds, this rigidity gives the sail strength and retains proper shape instead of stretching. But in light winds, the sail material may be too much for the breeze to press into shape.
We can use gravity instead. By leaning the boat over toward the side the sails are on (i.e., the leeward side), the belly of the sails will dangle away from the mast and fall into shape. Heeling the boat is often easily accomplished by shifting the moveable ballast, ie crew. Next time you can feel some breeze on your face but your sails just hang there like old socks, shift those pirate booties to leeward, and things just might fall into place.