Many people who have not worked in a dentist's office or whom are not trained dentists have not heard the term "dental loupes". I'm sure they have seen them, however, but simply had no idea what they were called. This is essentially a device worn on the head over the eyes, either over clear lenses (people with no need for corrective lenses) or there are some that fit right over your native glasses as well, adjustable to your vision needs. This device is used to see into a patient's mouth at a higher resolution than is possible with the naked eye.
Dental loupes are used to assure accuracy when giving a diagnosis, and for doing precision work. Just like you'd use the proper tools in a surgery if you were a doctor, dentists use these devices too. In fact, dentists and doctors use a similar form of magnification lenses, as they both use them to see into small places, magnify, add light to, and assure that nothing goes wrong either in their diagnosis of the problem, or in fixing the problem once they've figured out what it is.
When it comes to the type of work a dentist does, the tiniest crack in your tooth can mark a big problem later. Missing this crack, for example, could possibly render you helpless to save the tooth later when the problem gets worse and you're not aware there even is a problem until the pain starts. Dental loupes have been used pretty much as long as the magnifying lenses have been in circulation. The devices used at first, of course, weren't as accurate or high quality as the ones used today, but they still were better than simply using the naked eye.
Some dental loupes are very high tech, going way beyond just magnifying the inside of your mouth. Some are hooked up to a computer where you essentially get another set of digital eyes to help the dentist see things not visible to the naked eye. There are ways of seeing beyond the normal spectrum of light or bouncing inferred light for example off the tooth where a computer can digitally analyze it in real time showing the dentist where the problem is/may be. It's pretty amazing what's out there, and it's only getting better!