The Watt (W) is a measurement of power, the rate at which you consume energy. Energy is thus measured in Wh, so in 10 hours a 100W lightbulb (if there are any left) would use 10 x 100 = 1000 Wh = 1kWh (k, kilo, indicates 1000).
Your usage histograms show energy consumption in kWh. If you have used 0.46 kWh in half an hour, the average power of the things using electricity during that half hour is 2 x 0.46 = 0.92 kW or 920 W. You may see this referred to as the power draw : “My hoover draws 1200 W at full blast” (you should see the power rating of every electrical item on the item itself). If I ran my hoover for half an hour, it would consume ½ h x 1200 W = 600 Wh = 0.6 kWh.
Your meter measures the amount of energy you use every half hour, so you can add all those figures up for the whole day to see approximately how much you used that day. That is what OVO bill you for.