The red flap under a turkey's chin is called a wattle. A turkey's wattle is simply bare skin, and it can change from red to gray and even to white or blue depending on the turkey's distress or excitement levels.
Every turkey has a wattle underneath its chin, and just as humans' features differ, turkeys' wattles come in various shapes and sizes depending on the turkey. For every turkey, though, the wattle turns bright red if the turkey is angry, upset or otherwise distressed. A turkey's wattle also turns bright red during courtship, a sign it is ready to mate.