You could be talking about any number of treatments. From what you described it may have been:
a) The application of a high concentration fluoride varnish
b) The application of an unfilled resin
c) The restoration of the eroded defect with a composite resin restoration
The best advice would be to speak with your dentist. They should be able to answer all of the questiosn that you have posted here.
Also, try to look at the causative factors rather than just treating the symptoms, to prevent further tooth tissue loss.