Grid parity is the point in time when the price of electricity from solar power (or another resource) becomes equal to the price of electricity from the (fossil-fuel based) electrical grid. Significant decreases in the price of installed solar in recent years has lead to grid parity in a number of countries as well as several states in the US.
According to the following map put together by the Union of Concerned Scientists, grid parity has already been reached or exceeded in 11 states in the US, and it is estimated that number will be up to 28 states by 2017:

http://www.dailyfinance.com/2014/12/31/2015-solar-energy/
It’s no surprise that sunny CA, AZ, TX are at grid parity given their high solar radiation levels, but its interesting to note that several northeast US states are also considered at grid parity, which likely reflects higher local grid electricity prices in those states than in other regions of the country.