Mean square convergence of Fourier series refers to the condition where the sequence of partial sums of the Fourier series converges to the function in the mean square sense. Specifically, this means that the integral of the square of the difference between the function and its Fourier series representation approaches zero as the number of terms increases. The discussion emphasizes the importance of this convergence in ensuring that the Fourier series accurately represents the original function in terms of energy. Additionally, it highlights that mean square convergence is a stronger form of convergence than pointwise convergence. Understanding this concept is crucial for applications in signal processing and harmonic analysis.