Gelatin is a protein that dissolves in hot liquids and gels when cold. It is used to set light custards such as panna cotta among various other uses both sweet and savory.
The natural gelatin contained in meat and bones is what causes cold broth consommé or aspic to set.
Many of home cooks don’t like gelatin because if overused, it makes things rubbery. It’s best used in the smallest amount needed to get a liquid to set, about half the amount specified on the package, which says that one packet will set 1 cup liquid. In fact, one packet will barely set, which generally what you want, 2 cups of liquid.
When using powdered gelatin, soften it in about 3 tablespoons cold water per packet before adding it to hot or warm liquids.
Some recipes call for sheet gelatin, which happens to be the preferred form in Europe. When using sheet gelatin, soak it first in cold water until it becomes soft. It’s difficult to arrive at equivalents between sheet gelatin and powdered gelatin because different brands of sheet gelatin contain different amounts of gelatin per sheet.