LUTS stands for Look Up Table. They are small files which can be used in colour grading software for video or photography software to alter the colour and appearance of an image. As the name implies, a LUT consists of a table of values, which references different values of colour, and for each of those values, it gives you a destination value. In other words for a particular shad of blue in the source image, the LUT tells it to modify that shade based on the value in the table. In other worlds, you can say, this shade of light blue should be this shade of purple and so on. In some respects they are similar to Lightroom presets or colour profiles, but have a much wider range of usage.