A non-dispatchable source of electricity generates electrical energy but cannot be turned on or off in order to meet societies fluctuating electricity needs. It is the opposite of dispatchable sources of electricity which are very flexible, being able to change their output fairly quickly in order to meet electricity demands. Non-dispatchable electricity sources are often highly intermittent, which means that they are not continuously available due to factors that cannot be controlled (ex. the weather). There are many different types of non-dispatchable sources such as tidal power and wave power, but two main types that contribute noticeably to the electrical grid: Solar power and wind power.
Solar power is intermittent and most often non-dispatchable. Solar energy to the Earth is not the same in all locations, and is also affected by cloud cover. The main reason that solar power is non-dispatchable is the fact that the Sun does not shine for all hours of the day in a given location. Therefore, it is impossible to make consistent use of this resource for electricity generation without the use of electricity storage.
Wind power is considered highly intermittent and non-dispatchable because it is a variable power source, meaning that its electrical output depends on many factors, such as wind speed, air density, turbine characteristics, and more. All of these factors also change depending on location of the site. Wind speed must also be in a certain range (depending on the turbine), above 3.5 m/s in order to generate electricity, and below 25 m/s to avoid damage to the turbine. When analyzing a wind farm's intermittency it would make sense that the reliability would somewhat increase, but this actually doesn't appear to be the case. The difficulty comes from the fact that electricity supply must exactly match electricity demand every second. More turbines means that it is more likely that some electricity will be produced, but not necessarily that the right amount will be.