Mining companies dig up — or blast away — huge chunks of mountains to get at the coal buried beneath them.
All that material, which can include poisonous heavy metals, ends up scattered around the mining sites, and can potentially enter local streams. And once it's in the streams, it can enter the water supply, potentially threatening the health of local populations.
Under Obama, the EPA created the Stream Protection Rule, which would have required mining companies to study the health of local streams before and during mining activities, and then restore them to their original condition. Mining companies objected to the rule as too expensive.
Trump promised to kill the rule in his post-election environmental manifesto, and it was one of the easier targets. It had gone through a required notice and comment period, but was not yet in force.
Using a 1996 law called the Congressional Review Act that applies to regulations in that limbo period, the house voted to kill the Stream Protection Rule — along with four other new regulations — before it went into effect. The Senate also voted to remove the rule, Trump then signed the rule, and the Stream Protection Rule was no more.