Welcome! :)
The minimum payout is the minimum of GRC that the pool needs to owe you in order to make a payout. In other words: If you choose a higher value (let's say 100), your GRC will stay on grcpool until you are above that value (100). Then there will be the payout.
The benefit of a higher minimum payout is, that you do not lose that much on transaction fees (they are fixed at 0.005 GRC per payout, which is VERY low though).
I set my minimum payout to 10.

Hope that answers your question :)