Its mathematical procedure, performed in data (usually digital images or music), which converts it from one domain (for example, time) to another (frequency), usually performs Fourier or Laplace transforms.
In the new domain, data can be more easily processed, for lossy compression, noise removal, sharpness, etc. After editing, the data is converted back to the original domain.
In music, such Transforms are used to compress MP3s, removing higher (and almost inaudible) frequencies that take up a lot of space.
In images, it is mainly used for noise filters (sharpness, blur, etc.) and watermarks.