To stretch and fold bread dough, lift a portion gently, extend it without tearing, and fold it over the rest. Repeat around the dough as the recipe directs. Rest periods between sets allow the dough to relax.
Folding develops and organizes the gluten network during bulk fermentation. It is especially useful for wet doughs. Stop stretching when the dough resists rather than forcing it until it tears.
Reference: Further reading.