Day trading is the practice of buying and selling stocks (usually) all within the same trading day. In theory, day traders should not hold stocks over night. The problem with day trading is the transaction costs. Constant buying and selling will eat your returns, and even with low commissions, the bid-ask spread will still erode your wealth.
That said, I have to say I was impressed by this one day trader's set up. I only hope he makes enough to cover his light bill.