Marmite is concentrated yeast paste from the UK and it tastes exactly like that. If you took fresh baked bread and captured the smell from it, and did the same from a hundred thousand other loaves, until you had a sticky black sludge that was so intensely flavorful that it hurt to eat, and then mixed it with way too much salt you'd get Marmite. I don't think it's possible to describe it accurately and actually have it sound like a pleasant experience.
The first time I tasted it I hated it. I met some Brits who had come to Portsmouth, NH for summer work and they needed a place to stay for a few days so I put them up. By way of thanks, they shared their Marmite with me. It was disgusting. Maybe the worst thing I ever tasted. I shuddered for hours. Maybe there's something infectious about it because years later I had a craving for it. I hunted down a bottle and spread a thin layer on a toasted bagel with plenty of butter. It was amazing. Like good coffee or fine Scotch, you taste it with your nose as much as your tongue. It awakens the senses and charges the mind.
Try it. You probably won't like it. Some foods are worth the effort, though. Good coffee and fine Scotch taste pretty awful the first time, too, but they can offer a lifetime of enjoyment once you learn to appreciate them. Maybe Marmite will become a food you crave, too?