When a vault is farming, it periodically sells its target tokens and buy back the principal tokens and YFII before distributing the yields to vault users. For example, a TUSD vault farming CRV will periodically sell CRV and buy TUSD with 90% of the CRV yields and buy YFII with the left 10% and then distribute these tokens according to the rules stipulated by YIP-5. This process is called harvesting and an active vault needs to harvest from time to time.
Anyone can invoke the Harvest function to collect yields into the vault and in return get 1% of the yield from that batch. It may cost gas of approximately 0.1 ETH. Frequent harvesting can help reduce the slippage during token swaps and thus increase the whole income of the Vault. Please keep an eye on the current yield and also check if others have already invoked the function recently, or the rewards won't cover your gas fee.
You are supposed to be familiar with how smart contracts work. Currently there is no front-end for the harvest function so you have to interact with the contracts using Etherscan or other CLI tools you like.
For example, if you want to harvest for the USDT vault, go to the Write Contract page on Etherscan of the strategy address. Then click Connect to Web3 to connect with your wallet.
Find the 3rd function harvest and click Write. Accept the transaction in Metamask and when it's confirmed you have successfully helped harvest.
Here's a list of strategy contract addresses for each vault:
USDT: 0xe2df4c46acabb1cdb446351d6b24727944a5bfcc
yCRV: 0x898828957133d4c50030a5A2D55Ca370915E6A77
DAI: 0xbDD4a57c5EE8558370bb661d29a979657D81258e
TUSD: 0x30aE128ebCdec11F62cB3fa9C6a0E8269a9AF686