This chart is measuring kWh/m²/day. So for
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1,387kW is the total energy that is received for one year, for a one meter heliostat in NJ.
3.8 kWh/m²/day * 365 day = 1,387kWatts
If you could take this 1,387kW and convert it straight to the electrical energy it would power your house for two months.