Fourier series dissembles periodical functions into the sum of functions from sinuses and cosinuses. We will define the theory of Fourier series, which we will transform with the help of Fourier coefficients. We will list types of functions, like periodical, aperiodical function, and odd and steam functions, and their differences. We will list a few types of measuring sensors, and adduce the types of data they collect. With usage of measuring sensors, we collect data, or variables of various physical or chemical values of natural phenomena. We will list their classification, and say something about each, and adduce some of their examples. That data, or numeric chemical or physical variables written in digital shape, can be written in the form of periodic functions. The periodic functions can then be written in the form of sums and harmonic series of sinus and cosinus from which the Fourier series are formed. The periodic functions are called signals. We will induce some of the types of signals, among which we will define analog and digital signals. The difference between thise two types of signals is that the analog signal is a time continuous signal which amplitude changes with time constantly and continual with time, whereas the digital signal is formed by discrete functions which we get by sampling of the record of the state of analog signal in certain time pitches which length depends on the frequency of the signal. In other words, for getting the digital signal from the analog signal, we need to use Fast Fourier Transformation, or FFT. We will list other types of signals also, which will be explained and few examples will be shown. From a series of time functions, we get spectres. We will explain what spectres are and list few of their examples.