Soluble fiber is dietary fiber that dissolves in water and forms a gel-like substance in the digestive system, as opposed to insoluble fiber, which passes through largely intact. It's found in oats, barley, beans, lentils, peas, apples, citrus fruit, and psyllium; pectin, guar gum, and oat or barley beta-glucan are its main food-grade forms.
In the kitchen, soluble fiber acts as a thickener and gelling agent: pectin sets jams and jellies, and guar gum thickens sauces and gravies. Because it absorbs liquid, adding fiber-rich ingredients to soups, stews, or smoothies also thickens them, so recipes calling for it should be followed carefully to avoid over-thickening.
Soluble fiber is also associated with digestive and cardiovascular health benefits, though those are nutritional rather than culinary considerations.