As has been said many times by experts, the US electric grid is the largest machine in the world. More than 7,300 power plants are connected by 6.8 million miles of power lines. If one region is running low on power, electricity can be sent from hundreds of miles away to make up the difference. Even still, there may be times when there just isn't enough power to go around, so utility companies have to make tough choices to protect the electric grid.
Rolling blackouts are temporary outages that move from one section of the electric grid to another, which prevents the grid from being overloaded and experiencing a catastrophic failure. This can happen when too much power is being demanded at once, such as during extreme cold spells or heat waves, or when power plants and other generating facilities stop producing power unexpectedly. While they can be inconvenient and frustrating, rolling blackouts are a tool electric utilities use to keep the electric grid from collapsing and causing widespread outages that could take days or weeks to restore.
Simply put, small power outages are created to reduce the strain on the grid, and as one outage is restored another small outage is created in a different section of the grid. These outages "roll" throughout the grid until the energy crisis is over.