Registration refers to the alignment of prints when using multiple colors. Registration can be considered as the method of making a correlation of the colors that overlap in one image. Registration happens to be of many different types and styles. Many of these styles involve alignment of some particular marks.
The purpose of registration is to reduce the blur that comes when more than one color is there in an image while printing. Registration ensures that each color is printed separately so that no mixing occurs, and each color comes out without a blur. In other words, it perfectly lines up all the colors.