What is Marmite?

This question takes brevity to an extreme that might be described as unhelpful at best.

Here are a few alternative answers, which answer the question perfectly well:

It's a spread that's lovely on toast.

It's something you either love or hate, and has become a byword for such. For example: "Is IKEA the Marmite of retailers?."

It's a by-product of the brewing industry.

It's a food product, given the name of a kind of French cooking pot.

It's a rich source of vitamins.

The question would have been better worded thus: "What is marmite made from?"