What are duck curves? What do ducks have to do with solar energy?

Source: http://2greenenergy.com/2015/04/14/demand-for-energy-storage-in-a-changing-world/

When do you need the most energy? You probably don't need much when you are sleeping (which is why many do laundry at night, it can be cheaper). You probably need more when you first get home from work or school, when you turn your air conditioner on (or turn it up, or even, naturally, as you get into the heat of the day). Solar cells produce most of their energy during the day when the sun is shining. Distributed systems like wind power also have this issue, although they produce the most energy at different times of day.
A duck curve shows the net electrical load as a function of time. This is used by utilities to determine how much additional power needs to be produced to meet demand. The "duck" shape is obtained because solar energy produces most of its energy during the day, reducing the net load.

Notice that near the neck of the duck, the curve is becoming increasingly sharp. This is difficult for utilities to manage. They must quickly turn on other power plants to meet demand. A variety of technologies can be used to even out this curve. One can use batteries or capacitors to store energy generated during peak solar production hours, which can then be released in a more controlled fashion. One issue with this is that batteries tend to be expensive (although prices are coming down.) One can inexpensively pump water up a hill using energy supplied during peak hours and then allow it to flow down, capturing the energy during non-peak hours. Another way to handle this is to use other energy sources that have a different cycle, such as wind, which would tend to pick up right when solar energy would be going down and drop off when solar energy was turning on. One can also heat molten salts during peak hours and then extract the heat during non-peak hours, leveling the loads. People have even suggested (and attempted) storing energy in superconducting magnetic coils, the downside being refrigeration requirements!