all you need to know that their taker trading fee is 0.25% or 0.3% (depending on pair).
You will NEVER see a rebate, so it doesn't matter for you.
But if you still would like to know:
It simply means, that if you trade a very big volume within 30 days, your fee will be lower. So eg instead of 0.25 you get 0.24 or even lower.
Most exchanges have a list which tells you "if you trade over 100 BTC in 30 days, the fee will reduce by x". But gdax say instead "if your 30 day trading volume is 5% of the total exchange volume, the fee will be reduced by x". But since the total volume is that incredible high, you must trade hundreds or even thousands of BTC to lower your fee. And that is why you will never get rebate