A vault is a smart contract strategy that uses an algorithm that is programmed to automatically take the multi-step process of auto-compounding your assets. To give you an idea of how they work, traditionally users add LP tokens to yield farms, and are rewarded with a reward token.
If you deposit your LP tokens in a vault, every time the "Compound" button is pressed, the vault will harvest your pending rewards, sell half of your reward tokens, buy the paired assets with the proceeds, use those two tokens to add liquidity and then increase your LP token amount, automatically.
Compound on the single assets will harvest the pending rewards, sell only the amount required based on the fee structure, and redeposit the majority of the harvest.
Jetswap's vaults reward users with auto-compounded rewards that are much higher than the non-compounded rewards offered by yield farms themselves. This saves users time, gas fees, and protects their gains.