The text is a comprehensive overview of the Fast Fourier Transform (FFT), explaining its function as a method to convert signals from the time domain to the frequency domain, akin to a prism splitting light into colours. It extensively details the crucial applications of FFT within the power system, particularly for the Transmission System Operator Energinet, focusing on how it is used to analyse power quality and system stability. Specific uses include the detection and quantification of harmonic distortion and noise, the monitoring of system oscillations (such as inter-area swings), and the analysis of transient events like faults and lightning strikes. Crucially, the source highlights that the computational speed of the FFT algorithm allows for near real-time monitoring, enabling both automated and manual interventions to maintain grid health and ensuring the results are used for strategic decision-making and control system adjustments.