Perhaps the least appetizing of culinary terms, sweating means cooking food, usually chopped vegetables, in a little oil without browning it.
Most foods contain some natural oil or moisture, which is where a lot of their flavor is stored. When they’re cooked, the heat makes the food release that moisture and flavor. As the food softens, the moisture is released onto the surface of the food, coating it and giving it a sheen that makes it look as though it’s sweating. Sweating vegetables is typically done at the start of a recipe so that the softened vegetables and the flavor they release serve as a delicious base for the rest of your dish.
Some recipes use the terms sweat and saute interchangeably, but they’re quite different. Sautéing is done at a higher temperature with a goal of browning or finishing a dish, while sweating is a step in a longer cooking process.