This is the redeemable amount of Ether for your GAIN tokens. It is determined by the ratio between redeemable GAIN tokens and Ether held in the Gain Pool.
There are 10,000,000 redeemable GAIN.
100,000 (1%) of all redeemable GAIN tokens are sent to the Harvest contract.
The Gain Pool has a balance of 50,000 Ether at the close of the quarter.
In this case, the Spot Harvesting Rate would be:
200 = 10,000,000 GAIN / 50,000 Ether
Meaning that for every 200 GAIN tokens sent to the Harvesting contract, 1 Ether will be exchanged.