These are small instruments, corded and cordless, that purport to discover carious lesions in unrestored teeth when these same lesions cannot be seen on a radiograph and/or found during a clinical exam with a mirror and explorer. They can also provide an unbiased confirmation for a patient that your clinical exam is accurate and you are using the most current diagnostic aids.
However, none of these products is 100% accurate and following their readings unequivocally can result in overtreatment.