Grid parity can be defined as the point in time where an alternative source of energy can produce electricity for consumers at the same rate as the conventional sources of power. This term is often associated with solar power.
To understand grid parity better, it is important to understand another term associated with it, known as "levelized cost of electricity". It can in simple terms be defined as the ratio of sum of costs incurred for a power generation system in it’s lifetime to the sum of power generated by the same system over it’s lifetime.
Grid parity is determined by comparing the levelized cost of energy produced by the renewable energy source and the levelized cost of the utility grid. Additionally, grid parity depends on a lot of other factors like the location, utility grid, etc.