A staff member runs a device over your face and it returns a reading of things like moisture, sebum, pores, wrinkles, pigmentation, redness and elasticity. You get the numbers on a screen or a printout, then a recommendation of products in that store which match the reading. It takes roughly 10 to 15 minutes.
It is a retail service, not a medical one. The device measures the surface of your skin at one moment, on one day, in shop lighting, after you have been outside walking around Seoul. That is worth remembering before you take any single number too seriously.
If you want something actually diagnosed, that is a dermatologist, not a shop counter.