That's a visual depiction of your balance with Bitstamp. The fact that it goes from 0 to 0.5 means that Bitstamp can owe you anywhere from 0 to 0.5 BTC, becuase that's how much you trust them with. (If Bitstamp also trusted you, the left end would be negative, because you could be in debt to Bitstamp.) The green line shows how much of that trust they're using. As you receive Bitstamp BTC IOU's, the bar will fill up; as you spend them, it will empty. When it's full, that line of credit is maxed out, and you won't be able to receive any more Bitstamp BTC until you increase the trust limit or spend some you already have.