Marmite and vegemite are both yeast extracts that have a deep umami flavor. Folks in England use marmite and in Australia use vegemite to spread on toast. They're very intensely flavored when you eat them that way. But when you add a teaspoon to a large batch of gravy, you don't end up tasting it per se. It just adds a lovely depth of flavor that complements the meatiness of the turkey and traditional Thanksgiving sides.