The Duck curve is the name given to the pattern of supply and demand for electricity in a system with a lot of solar power capacity, such as in California where the term originated. Solar output peaks in the middle of the day then declines rapidly as the sun sinks in the late afternoon/evening, at the same time that demand is increasing as people return from work and demand electricity for cooking, air conditioning etc. Generators such as hydro and gas-fired power stations have to increase their output rapidly to keep up with the declining solar generation and increasing demand. The rate at which they have to increase their output is known as slew rate and is measured in units of watts/time, e.g. gigawatts per hour.
"The Duck Pond" by Bonnie Marini, PhD in Power Engineering  on 22nd Mar 2016
"What the duck curve tells us about managing a green grid" from California ISO  2016
"More renewables? Watch out for the Duck Curve" by Judith Curry  (5 Nov 2014) discusses the duck curve in the context of managing grid-connected variable renewables generally.
"What’s the Point of an Electricity Storage Mandate?" by Catherine Wolfram in the Energy at HAAS blog on 29 July 2013  discusses the duck curve and suggests an economic pricing mechanism for addressing it.