Put simply this refers to the number of colours that a scanner is capable of recreating when copying an image or negative. Each pixel requires as standard 24bits to create true colour. Most of the basic scanners will have this. Many other scanners will have from between 30- 40bits but still actually only output 24. The reason for this is that the scanner performs a process where by it selects the best possible matching colours from the larger range (or palette) available to it giving a better range of colour in your scan.