A 2x reward farm allows a user to earn $SUSHI & an addition token as a reward when their SLP tokens are staked in it. These are extra rewards added on top of the trading fees earned from providing liquidity.

These farm contracts are named Masterchefv2 (or Minichefv2 on L2 & sidechains).

This allows other projects that are listed on Sushiswap to incentivize their token holders to provide liquidity for that specific token.

You can learn more about Masterchef contracts here.