Marmite: either you love the stuff or hate it. ("Love" meaning you are British and grew up eating it, and "hate" meaning you are everyone else.)
Yet even though Marmite is an acquired taste, it's an ingredient a surprising number of people keep in their kitchen cabinets. Why? Because it excels at adding and amping up umami (that savory, delicious "fifth taste" that makes things like cheese, mushrooms, beef, and soy sauce taste so mouth-smackingly good) to all kinds of dishes. It's also vegan (but not gluten free), so you can serve it to almost anyone.
So what is Marmite exactly? It's a goopy brown paste made from yeast extract that has a deeply funky and intense flavor. It is savory, salty, and pungent, with an underlying tone of almost honey-like sweetness and a little hint of malt. If you make the mistake of eating a teaspoonful straight, as I once did, your nostrils and eyeballs will bulge out like an old-time cartoon character.
Traditionally, Marmite (and its Australian cousin, Vegemite, also a yeast extract spread) were eaten on toast. It's actually pretty tasty in this form, especially if you put some butter or cheese on your toast first. Having a rich, fatty component tones down Marmite's harsh edges and helps its savory, mouth-watering qualities shine.
However, Marmite is most useful as a cooking ingredient. J. Kenji López-Alt calls it an umami bomb and uses it in many recipes. I like to add a dollop to pasta sauces, or to turn up the volume on sauces, glazes, and gravies.
If you're unsure of how to cook with Marmite, check out this compendium of recipes, and keep in mind that if you happen to have Vegemite handy, you can most likely use it in the same recipes.
Plus, since neither are costly (a jar of Marmite or Vegemite goes for about $3 to $4 every 100 grams or so), you can easily experiment with different foods. If you're an old hand at Marmite, you might want to try "limited edition" Marmite XO, which has been aged for more intense flavor.