The Harvest Guardian, Scarecrow, is an innovation to prevent arbitrage bots from stealing from investors. By limiting the frequency of harvest, it prevents bots from continuously harvesting and dumping, which leads to diminishing the financial returns from the community.
For example, the harvest lockup of the ARENA-BUSD farm is 2 hours. It means that farmers who stake in the ARENA-BUSD farm can only harvest (claim their rewards from farming) every 2 hours.
You can check the harvest lockup on each farm card.
To clarify, the harvest lockup only locks users' farming rewards. The tokens and LP tokens staked in farms can be withdrawn anytime.