To add to what others said, it can be any kind of pernicious pollution, including but not limited to: nuclear waste leakage, creosote from wood preservation, PCBs from power distribution equipment, toluenes (including TNT), aromatic hydrocarbons and other nasty solvents from industrial processes, to just people dumping bad batches of chemicals for years without getting caught, often once the money runs out, at which point there's no money to pay for cleanup and the company goes bankrupt.

So then the Feds have to step in and pay for it.

A number of these compounds have benzene rings in them, or nasty chlorine or fluorine bonds. Supercritical water, injected with oxygen, is literally liquid fire. You can fully oxidize anything that can be oxidized, and unlike incineration most of the product is in aqueous solution at the end where it can be filtered and collected instead of ejected into the atmosphere to become someone else's problem.

That’s a typo, it’s supposed to say “superfun.”

Just kidding, superfund is a big pool of money that was created to clean up toxic waste sites around the country.

The US EPA, the federal Environmental Protection Agency has a large fund of money -- literally the "super fund" -- set aside to address very large, high impact environmental disasters or issues.

In other words, a superfund site is an environmental disaster site targeted for cleanup by the FedGov.