This lecture covers the calculation and interpretation of trigonometric Fourier series, including the orthogonality of co-sinusoids, the spectrum of periodic signals, and the concept of discrete spectrum. It also delves into the Hilbert interpretation, the Hilbert space of finite duration and energy signals, and the associated dot product. The lecture further explores the generalized Fourier series, the complex case of Fourier series, and the polar representation of Fourier coefficients.