Suppose you generate energy at your house with a wind turbine, or a solar cell array, or from a water wheel, personal nuclear plant, or whatever. All these things are going to at some point generate more power than you can use at that instant, so you can use batteries to store it, for the night time, or when there's no wind, etc.
Or you can sell the surplus energy, thus saving yourself the battery cost (at the price of being at the grid's mercy). In this way you get relief from energy prices without having to invest in as much infrastructure like batteries and inverters. It's my understanding that in most locations (I'm in Texas), somehow, the local electric company HAS to buy your excess power from you, because the distribution grid is seen to be a public service. But how much do you make? Do they HAVE to pay you a certain amount? I pay 14 cents a kWh. If I sell electricity back, do I make that, or do I make less? Does it matter how much I sell back? Does it matter if it's solar, or wind or coal (I imagine subsidys are at work). Plus, in TX, I don't really have a 'local electric company'; I can buy electrical service from many different companies. I wonder how this effects things?
