Sunday night, tragedy struck a Las Vegas country music festival as a gunman opened fire from above.

Fifty-eight people lost their lives and over 500 were injured in the largest mass shooting in United States’ history.

The attacker was on the 32nd floor of the Mandalay Bay Resort and Casino when he rapidly drowned festival-goers in bullets. Twenty-three firearms were recovered from his suite, along with high capacity magazines and 12 bump-fire stocks, or “bump stocks.”

But what in the world is a “bump stock”?

“Bump stocks” are devices that allow semi-automatic weapons to mimic automatics by allowing the user to fire more rapidly. Semi-automatic firearms require the trigger to be pulled after each shot, but with a “bump stock” attached, multiple shots can be fired after holding the trigger once.

Essentially, a bump-fire stock “bumps” the rate of ammunition fired when the trigger is pulled.

The stocks attach to the buttstock and pistol grip of an AR-15 rifle.

They also can cost as little as $50, and are legal in the United States.

However, on Wednesday, Senator Dianne Feinstein (D-CA) introduced legislation to ban bump-fire stocks.

“There is no better way to honor the 59 people who were slaughtered than to take action,” she said.

On Thursday, the National Rifle Association released a statement saying the organization believed certain modification devices, like the bump-fire stock, should be “subject to additional regulations.”