Maintenance margin is what the exchange charges the clearing place or whomever to hold your positions into the market close.
Day-trading margin is a discount passed unto you, the holder of the contract(s), by the broker, and they assume the rest of the risk associated with your positions you currently have. So if you have a position that requires $10K margin and you have $5K in your bank, but the broker says you need only $500 to hold it, then they are fronting the other $4,500 on your behalf.
That is the very very basics of it.