This refers to an abrasive. Many different files and buffers are used by a nail professional for different purposes. The ‘grit’ denotes the level of abrasiveness. The lower the number the more abrasive the file or buffer; the higher the number the less abrasive. 100 or 180 grits is commonly used to shape and refine a coating or overlay. 240 is the lowest grit that should be used on a natural nail as any lower can cause too much damage.