A gauge is a governance mechanism allowing you to use your voting power to allocate your share of governance where you desire in a specific way. In Marinade, you can obtain voting power by locking your MNDE into a Marinade NFT Chef.
For example, let's imagine that there are only 1000 locked MNDE used in the Validators gauges, and those MNDE control the delegation of 2M SOL. If you own 10 locked MNDE in the gauges, you could allocate 1% (or 20,000 SOL) to the validator of your choice. This principle can be applied to control any type of allocation in a decentralized way. In Marinade, gauges currently allow the community to control two things: 20% of the Marinade stake in the Validators gauges and the distribution of 1M MNDE per week as liquidity mining incentives in the Liquidity mining gauges.